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README.md

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+# Tech Interview Handbook
+
+Handbook to help you ace your next technical interview, with a focus on algorithms and the front end domain. System design questions are in-progress.
+
+This handbook is pretty new and help from you in contributing content would be very much appreciated!
+
+## Motivations
+
+While there are many awesome books like [Cracking the Coding Interview](http://www.crackingthecodinginterview.com/) and interview-related repositories on Github that contain a lot of algorithm questions, for more domain-specific and non-software engineering questions, there seems to be a lack in resources. This handbook aims to cover questions beyond the algorithm coding questions.
+
+## Contents
+
+- **[Algorithms Questions](algorithms)**
+  - Questions categorized by topic.
+- **[Design Questions](design)**
+- **[Domain-specific Questions](domain)**
+- **[Front End Study Notes](front-end)**
+  - Summarized notes on the various aspects of front end.
+  - [Front End Job Interview Questions and Answers](front-end/interview-questions.md) ⭐
+- **[Non-Technical Tips](non-technical)**
+  - Random non-technical tips that cover behavioral and psychological aspects, interview formats and "Do you have any questions for me?".
+  - [Behavioral Questions](non-technical/behavioral.md)
+  - [Interview Formats](non-technical/format.md)
+  - [Psychological Tricks](non-technical/psychological.md)
+  - [Questions to Ask](non-technical/questions-to-ask.md)
+  - [Negotiation Tips](non-technical/negotiation.md)
+- **[Utilities](utilities)**
+  - Snippets of algorithms/code that will help in coding questions.
+
+## Contributing
+
+There are no hard contributing guidelines at the moment as things are still in flux, might find a better approach to structure content, but we will figure out as we go along! Just contribute whatever that you think will help others. If you would like to contribute content for different domains, feel free to create an issue or submit a pull request and we can discuss further.

algorithms/README.md

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+Algorithm Questions
+==
+
+Here are some tips and questions you can think in your head or clarify with the interviewer. They may lead you to
+discover corner cases you might have missed out or even lead you towards the optimal approach!
+
+## Contents
+
+- [Array](array.md)
+- [Dynamic Programming and Memoization](dynamic-programming.md)
+- [Geometry](geometry.md)
+- [Graph](graph.md)
+- [Hash Table](hash-table.md)
+- [Heap](heap.md)
+- [Interval](interval.md)
+- [Linked List](linked-list.md)
+- [Math](math.md)
+- [Matrix](matrix.md)
+- [Object-Oriented Programming](oop.md)
+- [Permutation](permutation.md)
+- [Queue](queue.md)
+- [Sorting and Searching](sorting-searching.md)
+- [Stack](stack.md)
+- [String](string.md)
+- [Tree](tree.md)
+
+## Picking a Programming Language
+
+Before anything else, you need to pick a programming language to do your interviews in. Most companies will let you code in any language you want, the only exception I know being Google, where they only allow candidates to pick from Java, C++ or Python for their algorithmic coding interviews. Most of the time, I would recommend that you use a language that you are extremely familiar with rather than picking up a new language just for doing interviews because the company uses that language heavily.
+
+There are some languages which are more suitable than others for coding interviews and some languages you absolutely want to avoid. From my experience interviewing as an interviewer, most candidates pick Python or Java. Other commonly seen languages include JavaScript, Ruby and C++. I would absolutely avoid lower level languages like C or Go, simply because they lack in many standard library functions and data structures.
+
+Personally, Python is my de facto choice for coding algorithms during interviews because it is succinct and has a pretty huge library of functions and data structures available. One of my top reasons for recommending Python is that it uses consistent APIs that operate on different data structures, such as `len()`, `for ... in ...` and slicing notation on sequences (strings/lists/tuples). Getting the last element in a sequence is `arr[-1]` and reversing it is simply `arr[::-1]`. You can achieve a lot with minimal syntax in Python.
+
+Java is a decent choice too but having to constantly declare types in your code means extra keystrokes which results in slower coding/typing speed. This issue will be more apparent when you have to write on a whiteboard during on-site interviews. The reasons for choosing/not choosing C++ are similar to Java. Ultimately, Python, Java and C++ are decent choices of languages. If you have been using Java at work for a while now and do not have time to be comfortably familiar with another language, I would recommend just sticking to Java instead of picking up Python from scratch just for interviews to avoid having to context switch between languages during work vs interviews. Most of the time, the bottleneck is in the thinking and not the writing.
+
+One exception to the convention of allowing you to "pick any programming language you want" is when you are interviewing for a domain-specific position, such as Front End/iOS/Android Engineer roles, in which you would need to be familiar with coding algorithms in JavaScript, Objective-C/Swift and Java respectively. If you need to use a data structure that the language does not support, such as a Queue or Heap in JavaScript, perhaps try asking the interviewer whether you can assume that you have a data structure that implements certain methods with specified time complexities. If the implementation of that data structure is not crucial to solving the problem, the interviewer will usually allow it. In reality, being aware of existing data structures and selecting the appropriate ones to tackle the problem at hand is more important than knowing the intricate implementation details.
+
+## Revise your CS101
+
+If you have been out of college for a while, it will be highly advisable to revise the CS fundamentals — Algorithms and Data Structures. Personally I prefer to revise as I practiced so I scanned through my college notes and revised the various algorithms as I worked on the algorithm problems from LeetCode and Cracking the Coding Interview.
+
+This [interviews repository](https://github.com/kdn251) by Kevin Naughton Jr. served as a quick refresher for me.
+
+## Mastery through Practice
+
+Next, gain familiarity and mastery of the algorithms and data structures in your chosen programming language:
+
+1. Practice coding algorithms using your chosen language. While [Cracking the Coding Interview](http://www.crackingthecodinginterview.com/) is a good resource for practice, I prefer being able to type code, run it and get instant feedback. There are various Online Judges such as [LeetCode](https://leetcode.com/), [HackerRank](https://www.hackerrank.com/) and [CodeForces](http://codeforces.com/) for you to practice questions online and get used to the language. From experience, LeetCode questions are the most similar to the kind of questions being asked in interviews whereas HackerRank and CodeForces questions are more similar to competitive programming questions. If you practice enough LeetCode questions, there is a good chance that you would have seen/done your actual interview question (or some variant) on LeetCode before.
+2. Learn and understand the time and space complexities of the common operations in your chosen language. For Python, this [page](https://wiki.python.org/moin/TimeComplexity) will come in handy. Also find out the underlying sorting algorithm that is being used in the language's `sort()` function and its time and space complexity (in Python its Timsort which is a hybrid sort). After completing a question on LeetCode, I usually add the time and space complexities of the written code as comments above the function body to remind myself to analyze the algorithm after I am done with the implementation.
+3. Read up on the recommended coding style for your language and stick to it. If you have chosen Python, refer to the PEP 8 Style Guide. If you have chosen Java, refer to Google’s Java Style Guide.
+4. Find out and be familiar with the common pitfalls and caveats of the language. If you point out them out during the interview and intelligently avoid falling into them, you will usually impress the interviewer and that results in bonus points in your feedback, regardless of whether the interviewer is familiar with the language or not.
+5. Gain a broad exposure to questions from various topics. In the second half of the article I mention algorithm topics and practice questions for each topic. Do around 100–200 LeetCode questions and you should be good.
+
+Practice, practice and more practice!
+
+## Phases of a Coding Interview
+
+Congratulations, you are ready to put your skills into practice! In a real coding interview, you will be given a technical question by the interviewer, write code in a real-time collaborative editor (phone screen) or on a whiteboard (on-site) to solve the problem within 30–45 minutes. This is where the real fun begins!
+
+Your interviewer will be looking out for signals that you fit the requirements of the role and it is up to you to display those signals to them. Initially it may feel weird to be talking while you are coding as most programmers do not have the habit of explaining out loud as they are typing code. However, it is hard for the interviewer to know what you are thinking just by looking at the code that you type. If you communicate your approach to the interviewer before you start coding, you can validate your approach with him and the both of you can agree upon an acceptable approach.
+
+**Before the Interview (Remote)**
+
+For phone screens/remote interviews, prepare paper and pen/pencil to jot down and visualize stuff. If you are given a question on trees and graphs, it usually helps if you draw out some examples of the data structure given in the question.
+
+Use earphones and make sure you are in a quiet environment. You definitely do not want to be holding a phone in one hand and only be able to type with the other. Try avoiding using speakers because if the echo is bad, communication is harder and repeating of words will just result in loss of valuable time.
+
+**Upon Getting the Question**
+
+Many candidates jump into coding the moment they hear the question. That is usually a big mistake. Take a moment to repeat the question back at the interviewer and make sure that you understand exactly what he is asking. If you misunderstood and when you repeat back the question, he'll clarify.
+
+Always seek clarification about the question upon hearing it even if it you think it is clear to you. You might discover something that you have missed out and it also sends a signal to the interviewer that you are a careful person who pays attention to details. Consider asking the following questions:
+
+- How big is the size of the input?
+- How big is the range of values?
+- What kind of values are there? Are there negative numbers? Floating points? Will there be empty inputs?
+- Are there duplicates within the input?
+- What are some extreme cases of the input?
+- How is the input stored? If you are given a dictionary of words, is it a list of strings or a Trie?
+
+After you have sufficiently clarified the scope and intention of the problem, explain your high level approach to the interviewer even if it is a naive solution. If you are stuck, consider various approaches and explain out loud why it will/will not work. Sometimes your interviewer might drop hints and lead you towards the right path.
+
+Start with a brute force approach, communicate it to the interviewer, explain the time and space complexity and why it is bad. It is unlikely that the brute force approach will be one that you will be coding. At this point, the interviewer will usually pop the dreaded "Can we do better?" question, meaning that he is looking for a more optimal approach. In my opinion, this is usually the hardest part of the interview. In general, look for repeated work and try to optimize them by potentially caching the calculated result somewhere and reference it later, rather than having to compute it all over again. There are some tips on tackling topic-specific questions that I dive into details below.
+
+Only start coding after you and your interviewer have agreed on an approach and has given you the green light.
+
+**Starting to Code**
+
+Write your code with good coding style. Reading code written by others is usually not an enjoyable task. Reading horribly-formatted code by others makes it worse. Your goal is to make your interviewer understand the code you have written so that he can quickly evaluate if your code does what you say it does and whether it solves the given problem. Use clear variable names, avoid single letter names unless they are for iteration. However, if you are coding on a whiteboard, you might not want to use extremely verbose variable names for the sake of reducing the amount you have to write.
+
+Always be explaining what you are currently writing/typing to the interviewer. This is not about literally reading out what you are typing to the interviewer. Talk about the section of the code you are currently implementing at a higher level, explain why it is written as such and what it is trying to achieve.
+
+When you copy and paste code, consider whether it is necessary. Sometimes it is, sometimes it is not. If you find yourself copying and pasting one large chunk of code spanning multiple lines, it is probably an indicator that you can refactor by containing those lines into a function. If it is just a single line you copied, usually it is fine. Do remember to change the respective variables in your copied line of code where relevant. Copy-paste errors are a common source of bugs even in day-to-day coding!
+
+**After Coding**
+
+After you have finished coding, do not immediately announce to the interviewer that you are done. In most cases, your code is usually not perfect and contains some bugs or syntax errors. What you need to do now is to review your code.
+
+Firstly, look through your code from start to finish as if it is the first time you are seeing it, as if it was written by someone else and you are trying to spot bugs in it. That’s exactly what your interviewer will be doing. Look through and fix any minor issues you may find.
+
+Next, come up with small test cases and step through the code (not your algorithm!) with those sample input. Interviewers like it when you read their mind and what they usually do after you have finished coding would be to get you to write tests. It is a huge plus if you write tests for your code even before prompts from them. You should be emulating a debugger when stepping through and jot down or say out the values of certain variables as you step through the lines of code.
+
+If there are huge duplicated chunks of code in your solution, it would be a good chance to refactor it and demonstrate to the interviewer that you are one who values code quality. Also look out for places where you can do [short-circuit evaluation](https://en.wikipedia.org/wiki/Short-circuit_evaluation).
+
+Lastly, give the time/space complexity of your code and explain why it is such. You can even annotate certain chunks of your code with the various time/space complexities to demonstrate your understanding of your code and the APIs of your chosen programming language. Explain any trade-offs in your current approach vs alternative approaches, possibly in terms of time/space.
+
+If your interviewer is happy with the solution, the interview usually ends here. It is also not uncommon that the interviewer asks you extension questions, such as how you would handle the problem if the whole input is too large to fit into memory, or if the input arrives as a stream. This is a common follow-up question at Google where they care a lot about scale. The answer is usually a divide-and-conquer approach — perform distributed processing of the data and only read certain chunks of the input from disk into memory, write the output back to disk and combine them later on.
+
+## Practicing via Mock Interviews
+
+Interviewing is a skill that you can get better at. The steps mentioned above can be rehearsed over and over again until you have fully internalized them and following those steps become second nature to you. A good way to practice is to find a friend to partner with and the both of you can take turns to interview each other.
+
+A great resource for practicing mock coding interviews would be [interviewing.io](https://interviewing.io/). interviewing.io provides free, anonymous practice technical interviews with Google and Facebook engineers, which can lead to real jobs and internships. By virtue of being anonymous during the interview, the inclusive interview process is de-biased and low risk. At the end of the interview, both interviewer and interviewees can provide feedback to each other for the purpose of improvement. Doing well in your mock interviews will unlock the jobs page and allow candidates to book interviews (also anonymously) with top companies like Uber, Lyft, Quora, Asana and more. For those who are totally new to technical interviews, you can even view a [demo interview](https://start.interviewing.io/interview/9hV9r4HEONf9/replay) on the site (requires sign in).
+
+I have used interviewing.io both as an interviewer and an interviewee and found the experience to be really great! [Aline Lerner](https://twitter.com/alinelernerLLC), the CEO and co-founder of interviewing.io and her team are passionate about revolutionizing the technical interview process and helping candidates to improve their skills at interviewing. She has also published a number of technical interview-related articles on the [interviewing.io blog](http://blog.interviewing.io/). interviewing.io is still in beta now but I recommend signing up as early as possible to increase the likelihood of getting an invite.
+
+Another platform that allows you to practice coding interviews is [Pramp](https://pramp.com/). Where interviewing.io matches potential job seekers with seasoned technical interviewers, Pramp differs takes a different approach. Pramp pairs you up with another peer who is also a job seeker and both of you take turns to assume the role of interviewer and interviewee. Pramp also prepares questions for you, along with suggested solutions and prompts to guide the interviewee. Personally, I do not really like Pramp's approach because if I were to interview someone, I would rather choose a question I am familiar with. Also, many users of the platform do not have the experience of being interviewers and that can result in a horrible interview experience. There was once where my matched peer, as the interviewer, did not have the right understanding of the question and attempted to lead me down the wrong path of solving the question.
+
+The next section dives deep into practical tips for specific topics of algorithms and data structures which appear frequently in coding questions. Many algorithm questions involve techniques that can be applied to questions of similar nature. The more techniques you have in your arsenal, the higher the chances of passing the interview. For each topic, there is also a list of recommended common questions which in my opinion is highly valuable for mastering the core concepts for the topic.
+
+## General Tips
+
+- Input validation:
+  - Always validate input first. Check for invalid/empty/negative/different type input. Never assume you are given the valid parameters. Alternatively, clarify with the interviewer whether you can assume valid input (usually yes), which can save you time from writing code that does input validation.
+- Are there any time/space complexity requirements/constraints?
+- Check corner cases:
+  - Check for off-by-one errors.
+  - In languages where there are no automatic type coercion, check that concatenation of values are of the same type: `int`/`str`/`list`.
+  - After finishing your code, use a few example inputs to test your solution.
+- Is the algorithm meant to be run multiple times, for example in a web server? If yes, the input is likely to be preprocess-able to improve the efficiency in each call.
+- Use a mix of functional and imperative programming paradigms:
+  - Write pure functions as much as possible.
+  - Pure functions are easier to reason about and can help to reduce bugs in your implementation.
+  - Avoid mutating the parameters passed into your function especially if they are passed by reference unless you are sure of what you are doing.
+  - However, functional programming is usually expensive in terms of space complexity because of non-mutation and the repeated allocation of new objects. On the other hand, imperative code is faster because you operate on existing objects. Hence you will need to achieve a balance between accuracy vs efficiency, by using the right amount of functional and imperative code where appropriate.
+  - Avoid relying on and mutating global variables. Global variables introduce state.
+  - If you have to rely on global variables, make sure that you do not mutate it by accident.
+- Generally, to improve the speed of a program, we can either choose a more appropriate data structure/algorithm or use more memory. It's a classic space/time tradeoff.
+- Data structures are your weapons. Choosing the right weapon for the right battle is the key to victory. Be very familiar about the strengths of each data structure and the time complexities for its various operations.
+- Data structures can be augmented to achieve efficient time complexities across different operations. For example, a hash map can be used together with a doubly-linked list to achieve O(1) time complexity for both the `get` and `put` operation in an [LRU cache](https://leetcode.com/problems/lru-cache/).
+- Hashmaps are probably the most commonly used data structure for algorithm questions. If you are stuck on a question, your last resort can be to enumerate through the possible data structures (thankfully there aren't that many of them) and consider whether each of them can be applied to the problem. This has worked for me sometimes.
+- If you are cutting corners in your code, state that out loud to your interviewer and say what you would do in a non-interview setting (no time constraints). E.g., I would write a regex to parse this string rather than using `split()` which does not cover all cases.
+
+## Sequence
+
+- Arrays and strings are considered sequences (a string is a sequence of characters). There are tips relevant for dealing with both arrays and strings which will be covered here.
+- Are there duplicate values in the sequence, would it affect the answer?
+- Check for sequence out of bounds.
+- Be mindful about slicing or concatenating sequences in your code. Typically, slicing and concatenating sequences require O(n) time. Use start and end indices to demarcate a subarray/substring where possible.
+- Sometimes you can traverse the sequence from the right rather than from the left.
+- Master the [sliding window technique](https://discuss.leetcode.com/topic/30941/here-is-a-10-line-template-that-can-solve-most-substring-problems) that applies to many substring/subarray problems.
+- When you are given two sequences to process, it is common to have one index per sequence to traverse/compare the both of them. For example, we use the same approach to merge two sorted arrays.
+- Corner cases:
+  - Empty sequence.
+  - Sequence with 1 or 2 elements.
+  - Sequence with repeated elements.
+
+## Array
+
+- Is the array sorted or partially sorted? If it is, some form of binary search should be possible. This also usually means that the interviewer is looking for a solution that is faster than O(n).
+- Can you sort the array? Sometimes sorting the array first may significantly simplify the problem. Make sure that the order of array elements do not need to be preserved before attempting a sort.
+- For questions where summation or multiplication of a subarray is involved, pre-computation using hashing or a prefix/suffix sum/product might be useful.
+- If you are given a sequence and the interviewer asks for O(1) space, it might be possible to use the array itself as a hash table. For example, if the array only has values from 1 to N, where N is the length of the array, negate the value at that index (minus one) to indicate presence of that number.
+
+#### Practice Questions
+
+- [Two Sum](https://leetcode.com/problems/two-sum/)
+- [Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/)
+- [Contains Duplicate](https://leetcode.com/problems/contains-duplicate/)
+- [Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/)
+- [Maximum Subarray](https://leetcode.com/problems/maximum-subarray/)
+- [Maximum Product Subarray](https://leetcode.com/problems/maximum-product-subarray/)
+- [Find Minimum in Rotated Sorted Array](https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/)
+- [Search in Rotated Sorted Array](https://leetcode.com/problems/search-in-rotated-sorted-array/)
+- [3Sum](https://leetcode.com/problems/3sum/)
+- [Container With Most Water](https://leetcode.com/problems/container-with-most-water/)
+
+## Binary
+
+- Questions involving binary representations and bitwise operations are asked sometimes and you must be absolutely familiar with how to convert a number from decimal form into binary form (and vice versa) in your programming language of choice.
+- Test k<sup>th</sup> bit is set: `num & (1 << k) != 0`.
+- Set k<sup>th</sup> bit: `num |= (1 << k)`.
+- Turn off k<sup>th</sup> bit: `num &= ~(1 << k)`.
+- Toggle the k<sup>th</sup> bit: `num ^= (1 << k)`.
+- To check if a number is a power of 2, `num & num - 1 == 0`.
+
+#### Practice Questions
+
+- [Sum of Two Integers](https://leetcode.com/problems/sum-of-two-integers/)
+- [Number of 1 Bits](https://leetcode.com/problems/number-of-1-bits/)
+- [Counting Bits](https://leetcode.com/problems/counting-bits/)
+- [Missing Number](https://leetcode.com/problems/missing-number/)
+- [Reverse Bits](https://leetcode.com/problems/reverse-bits/)
+
+## Dynamic Programming
+
+- Usually used to solve optimization problems.
+- Alaina Kafkes has written an [awesome post](https://medium.freecodecamp.org/demystifying-dynamic-programming-3efafb8d4296) on tackling DP problems.
+- The only way to get better at DP is to practice. It takes some amount of practice to be able to recognize that a problem can be solved by DP.
+- Sometimes you do not need to store the whole DP table in memory, the last two values or the last two rows of the matrix will suffice.
+
+#### Practice Questions
+
+- 0/1 Knapsack
+- [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/)
+- [Coin Change](https://leetcode.com/problems/coin-change/)
+- [Longest Increasing Subsequence](https://leetcode.com/problems/longest-increasing-subsequence/)
+- [Longest Common Subsequence]()
+- [Word Break Problem](https://leetcode.com/problems/word-break/)
+- [Combination Sum](https://leetcode.com/problems/combination-sum-iv/)
+- [House Robber](https://leetcode.com/problems/house-robber/) and [House Robber II](https://leetcode.com/problems/house-robber-ii/)
+- [Decode Ways](https://leetcode.com/problems/decode-ways/)
+- [Unique Paths](https://leetcode.com/problems/unique-paths/)
+- [Jump Game](https://leetcode.com/problems/jump-game/)
+
+## Geometry
+
+- When comparing euclidean distance between two pairs of points, using dx<sup>2</sup> + dy<sup>2</sup> is sufficient. It is unnecessary to square root the value.
+- To find out if two circles overlap, check that the distance between the two centers of the circles is less than the sum of their radii.
+
+## Graph
+
+- Be familiar with the various graph representations, graph search algorithms and their time and space complexities.
+- You can be given a list of edges and tasked to build your own graph from the edges to perform a traversal on. The common graph representations are:
+  - Adjacency matrix.
+  - Adjacency list.
+  - Hashmap of hashmaps.
+- Some questions look like they are trees but they are actually graphs. In that case you will have to handle cycles and keep a set of visited nodes when traversing.
+- Graph search algorithms:
+  - **Common** - Breadth-first Search, Depth-first Search
+  - **Uncommon** - Topological Sort, Dijkstra's algorithm
+  - **Rare** - Bellman-Ford algorithm, Floyd-Warshall algorithm, Prim's algorithm, Kruskal's algorithm
+- In coding interviews, graphs are commonly represented as 2-D matrices where cells are the nodes and each cell can traverse to its adjacent cells (up/down/left/right). Hence it is important that you be familiar with traversing a 2-D matrix. When recursively traversing the matrix, always ensure that your next position is within the boundary of the matrix. More tips for doing depth-first searches on a matrix can be found [here](https://discuss.leetcode.com/topic/66065/python-dfs-bests-85-tips-for-all-dfs-in-matrix-question/). A simple template for doing depth-first searches on a matrix goes like this:
+
+```py
+def traverse(matrix):
+  rows, cols = len(matrix), len(matrix[0])
+  visited = set()
+  directions = ((0, 1), (0, -1), (1, 0), (-1, 0))
+  def dfs(i, j):
+    if (i, j) in visited:
+      return
+    visited.add((i, j))
+    # Traverse neighbors
+    for direction in directions:
+      next_i, next_j = i + direction[0], j + direction[1]
+      if 0 <= next_i < rows and 0 <= next_j < cols: # Check boundary
+        # Add any other checking here ^
+        dfs(next_i, next_j)
+
+  for i in range(rows):
+    for j in range(cols):
+      dfs(i, j)
+```
+
+- Corner cases:
+  - Empty graph.
+  - Graph with one or two nodes.
+  - Disjoint graphs.
+  - Graph with cycles.
+
+#### Practice Questions
+
+- [Clone Graph](https://leetcode.com/problems/clone-graph/)
+- [Course Schedule](https://leetcode.com/problems/course-schedule/)
+- [Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)
+- [Pacific Atlantic Water Flow](https://leetcode.com/problems/pacific-atlantic-water-flow/)
+- [Number of Islands](https://leetcode.com/problems/number-of-islands/)
+- [Graph Valid Tree](https://leetcode.com/problems/graph-valid-tree/)
+- [Number of Connected Components in an Undirected Graph](https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/)
+- [Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/)
+
+## Interval
+
+- Interval questions are questions where you are given an array of two-element arrays (an interval) and the two values represent a start and an end value. Interval questions are considered part of the array family but they involve some common techniques hence they are extracted out to this special section of their own.
+- An example interval array: `[[1, 2], [4, 7]]`.
+- Interval questions can be tricky to those who have not tried them before because of the sheer number of cases to consider when they overlap.
+- Do clarify with the interviewer whether `[1, 2]` and `[2, 3]` are considered overlapping intervals as it affects how you will write your equality checks.
+- A common routine for interval questions is to sort the array of intervals by each interval's starting value.
+- Be familiar with writing code to check if two intervals overlap and merging two overlapping intervals:
+
+```py
+def is_overlap(a, b):
+  return a[0] < b[1] and b[0] < a[1]
+
+def merge_overlapping_intervals(a, b):
+  return [min(a[0], b[0]), max(a[1], b[1])]
+```
+
+- Corner cases:
+  - Single interval.
+  - Non-overlapping intervals.
+  - An interval totally consumed within another interval.
+  - Duplicate intervals.
+
+#### Practice Questions
+
+- [Insert Interval](https://leetcode.com/problems/insert-interval/)
+- [Merge Intervals](https://leetcode.com/problems/merge-intervals/)
+- [Meeting Rooms](https://leetcode.com/problems/meeting-rooms/) and [Meeting Rooms II](https://leetcode.com/problems/meeting-rooms-ii/)
+- [Non-overlapping Intervals](https://leetcode.com/problems/non-overlapping-intervals/)
+
+## Linked List
+
+- Like arrays, linked lists are used to represent sequential data. The benefit of linked lists is that insertion and deletion from anywhere in the list is O(1) whereas in arrays the following elements will have to be shifted.
+- Adding a dummy node at the head and/or tail might help to handle many edge cases where operations have to be performed at the head or the tail. The presence of dummy nodes essentially ensures that operations will never have be done on the head or the tail, thereby removing a lot of headache in writing conditional checks to dealing with null pointers. Be sure to remember to remove them at the end of the operation.
+- Sometimes linked lists problem can be solved without additional storage. Try to borrow ideas from reverse a linked list problem.
+- For deletion in linked lists, you can either modify the node values or change the node pointers. You might need to keep a reference to the previous element.
+- For partitioning linked lists, create two separate linked lists and join them back together.
+- Linked lists problems share similarity with array problems, think about how you would do it for an array and try to apply it to a linked list.
+- Two pointer approaches are also common for linked lists. For example:
+  - Getting the k<sup>th</sup> from last node - Have two pointers, where one is k nodes ahead of the other. When the node ahead reaches the end, the other node is k nodes behind.
+  - Detecting cycles - Have two pointers, where one pointer increments twice as much as the other, if the two pointers meet, means that there is a cycle.
+  - Getting the middle node - Have two pointers, where one pointer increments twice as much as the other. When the faster node reaches the end of the list, the slower node will be at the middle.
+- Be familiar with the following routines because many linked list questions make use of one or more of these routines in the solution:
+  - Counting the number of nodes in the linked list.
+  - Reversing a linked list in-place.
+  - Finding the middle node of the linked list using fast/slow pointers.
+  - Merging two lists together.
+- Corner cases:
+  - Single node.
+  - Two nodes.
+  - Cycle in linked list - Clarify whether there can be a cycle in the list? Usually the answer is no.
+
+#### Practice Questions
+
+- [Reverse a Linked List](https://leetcode.com/problems/reverse-linked-list/)
+- [Detect Cycle in a Linked List](https://leetcode.com/problems/linked-list-cycle/)
+- [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)
+- [Merge K Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)
+- [Remove Nth Node From End Of List](https://leetcode.com/problems/remove-nth-node-from-end-of-list/)
+- [Reorder List](https://leetcode.com/problems/reorder-list/)
+
+## Math
+
+- If code involves division or modulo, remember to check for division or modulo by 0 case.
+- When a question involves "a multiple of a number", perhaps modulo might be useful.
+- Check for and handle overflow/underflow if you are using a typed language like Java and C++. At the very least, mention that overflow/underflow is possible and ask whether you need to handle it.
+- Consider negative numbers and floating point numbers. This may sound obvious, but under interview pressure, many obvious cases go unnoticed.
+- If the question asks to implement an operator such as power, squareroot or division and want it to be faster than O(n), binary search is usually the approach to go.
+- Some common formulas:
+  - Sum of 1 to N = (n+1) * n/2
+  - Sum of GP = 2<sup>0</sup> + 2<sup>1</sup> + 2<sup>2</sup> + 2<sup>3</sup> + ... 2<sup>n</sup> = 2<sup>n+1</sup> - 1
+  - Permutations of N = N! / (N-K)!
+  - Combinations of N = N! / (K! * (N-K)!)
+
+#### Practice Questions
+
+- [Pow(x, n)](https://leetcode.com/problems/powx-n/)
+- [Sqrt(x)](https://leetcode.com/problems/sqrtx/)
+- [Integer to English Words](https://leetcode.com/problems/integer-to-english-words/)
+
+## Matrix
+
+- A matrix is a 2-dimensional array. Questions involving matrices are usually related to dynamic programming or graph traversal.
+- Corner cases:
+  - Empty matrix. Check that none of the arrays are 0 length.
+  - 1 x 1 matrix.
+  - Matrix with only one row or column.
+- For questions involving traversal or dynamic programming, you almost always want to make a copy of the matrix with the same dimensions that is initialized to empty values to store the visited state or dynamic programming table. Be familiar with such a routine:
+
+```py
+rows, cols = len(matrix), len(matrix[0])
+copy = [[0 for _ in range(cols)] for _ in range(rows)]
+```
+
+- Many grid-based games can be modeled as a matrix, such as Tic-Tac-Toe, Sudoku, Crossword, Connect 4, Battleship, etc. It is not uncommon to be asked to verify the winning condition of the game. For games like Tic-Tac-Toe, Connect 4 and Crosswords, where verification has to be done vertically and horizontally, one trick is to write code to verify the matrix for the horizontal cells, transpose the matrix and reuse the logic for horizontal verification to verify originally vertical cells (which are now horizontal).
+- Transposing a matrix in Python is simply:
+
+```py
+transposed_matrix = zip(*matrix)
+```
+
+#### Practice Questions
+
+- [Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes/)
+- [Spiral Matrix](https://leetcode.com/problems/spiral-matrix/)
+- [Rotate Image](https://leetcode.com/problems/rotate-image/)
+- [Word Search](https://leetcode.com/problems/word-search/)
+
+## Recursion
+
+- Remember to always define a base case so that your recursion will end.
+- Useful for permutation, generating all combinations and tree-based questions.
+- Be familiar with how to generate all permutations of a sequence as well as how to handle duplicates.
+- Recursion implicitly uses a stack. Hence all recursive approaches can be rewritten iteratively using a stack.
+- Beware of cases where the recursion level goes too deep and causes a stack overflow (the default limit in Python is 1000).
+- Recursion will never be O(1) space complexity because a stack is involved unless there is [tail-call optimization](https://stackoverflow.com/questions/310974/what-is-tail-call-optimization). Do find out if your chosen language supports tail-call optimization.
+
+#### Practice Questions
+
+- [Subsets](https://leetcode.com/problems/subsets/) and [Subsets II](https://leetcode.com/problems/subsets-ii/)
+- [Strobogrammatic Number II](https://leetcode.com/problems/strobogrammatic-number-ii/)
+
+## String
+
+- Please read the above tips on [Sequence](#sequence) because they apply to strings too.
+- Corner cases:
+  - Strings with only one distinct character.
+- Ask about input character set and case sensitivity. Usually the characters are limited to lower case Latin characters, i.e. a-z.
+- When you need to compare strings where the order isn't important (like anagram), you may consider using a hash map as a counter. If your language has a built-in `Counter` class like Python, ask to use that instead.
+- If you need to keep a counter of characters, a common mistake to make is to say that the space complexity required for the counter is O(n). The space required for a counter is O(1) not O(n), because the upper bound is the range of characters which is usually a fixed constant of 26 when the input set is just lower case Latin characters.
+- Common data structures for looking up strings efficiently:
+  - [Trie / Prefix Tree](https://www.wikiwand.com/en/Trie)
+  - [Suffix Tree](https://www.wikiwand.com/en/Suffix_tree)
+- Common string algorithms:
+  - [Rabin Karp](https://www.wikiwand.com/en/Rabin%E2%80%93Karp_algorithm) for efficient searching of substring using a rolling hash.
+  - [KMP](https://www.wikiwand.com/en/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm) for efficient searching of substring.
+
+#### Non-repeating Characters
+
+- Use a 26-bit bitmask to indicate which lower case latin characters are inside the string.
+
+```py
+mask = 0
+for c in set(word):
+  mask |= (1 << (ord(c) - ord('a')))
+```
+
+- To determine if two strings have common characters, perform `&` on the two bitmasks and if the result is non-zero, the two strings do have common characters: `mask_a & mask_b > 0`.
+
+#### Anagram
+
+- An anagram is direct word switch or word play, the result of rearranging the letters of a word or phrase to produce a new word or phrase, using all the original letters exactly once. In interviews, usually we are only bothered with words without spaces in them.
+- Determine if two strings are anagrams:
+  - Sorting both strings should produce the same resulting string.
+  - If we map each character to a prime number and we multiply each mapped number together, anagrams should have the same multiple (prime factor decomposition).
+  - Frequency counting of characters will help to determine if two strings are anagrams.
+
+#### Palindrome
+
+- A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward as forward, such as madam or racecar.
+- Ways to determine if a string is a palindrome:
+  - Reverse the string and it should be equal to itself.
+  - Have two pointers at the start and end of the string, move inwards till they meet. At any point in time the characters at both pointers should match.
+- The order of characters within the string matters, so hash maps are usually not helpful.
+- When a question is about counting the number of palindromes, a common trick is to have two pointer that move outwards, away from the middle. Note that palindromes can be even/odd length, and that for each middle pivot position, you would need to check twice, once including the character, and once without.
+  - For substrings, you can terminate early once there is no match.
+  - For subsequences, use dynamic programming as there are overlapping subproblems. Check out [this question](https://leetcode.com/problems/longest-palindromic-subsequence/).
+
+#### Practice Questions
+
+- [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/)
+- [Longest Repeating Character Replacement](https://leetcode.com/problems/longest-repeating-character-replacement/)
+- [Minimum Window Substring](https://leetcode.com/problems/minimum-window-substring/description/)
+- [Encode and Decode Strings](https://leetcode.com/problems/encode-and-decode-strings/)
+- [Valid Anagram](https://leetcode.com/problems/valid-anagram)
+- [Group Anagrams](https://leetcode.com/problems/group-anagrams/)
+- [Valid Parentheses](https://leetcode.com/problems/valid-parentheses)
+- [Valid Palindrome](https://leetcode.com/problems/valid-palindrome/)
+- [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/)
+- [Palindromic Substrings](https://leetcode.com/problems/palindromic-substrings/)
+
+## Tree
+
+- A tree is an undirected, connected, acyclic graph.
+- Recursion is a common approach for trees. When you notice the subtree problem can be used to solve the whole problem, you should try recursion.
+- When using recursion, always remember to check for the base case, usually where the node is `null`.
+- When you are asked to traverse a tree by level, use depth-first search.
+- Sometimes it is possible that your recursive function needs to return two values.
+- If the question involves summation of nodes along the way, be sure to check whether nodes can be negative.
+- You should be very familiar with writing pre-order, in-order and post-order traversal recursively. As an extension, challenge yourself by writing them iteratively. Sometimes interviewers do ask candidates for the iterative approach, especially if the candidate finishes writing the recursive approach too fast.
+- Corner cases:
+  - Empty tree.
+  - Single node.
+  - Two nodes.
+  - Very skewed tree (like a linked list).
+
+**Binary Tree**
+
+- In-order traversal of a binary tree is insufficient to uniquely serialize a tree. Pre-order or post-order traversal is also required.
+
+**Binary Search Tree**
+
+- In-order traversal of a BST will give you all elements in order.
+- Be very familiar with the properties of a BST and validating that a binary tree is a BST. This comes up more often than expected.
+- When a question involves a BST, the interviewer is usually looking for a solution which runs faster than O(n).
+
+#### Practice Questions
+
+- [Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)
+- [Same Tree](https://leetcode.com/problems/same-tree/)
+- [Invert/Flip Binary Tree](https://leetcode.com/problems/invert-binary-tree/)
+- [Binary Tree Maximum Path Sum](https://leetcode.com/problems/binary-tree-maximum-path-sum/)
+- [Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/)
+- [Serialize and Deserialize Binary Tree](https://leetcode.com/problems/serialize-and-deserialize-binary-tree/)
+- [Subtree of Another Tree](https://leetcode.com/problems/subtree-of-another-tree/)
+- [Construct Binary Tree from Preorder and Inorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/)
+- [Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)
+- [Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/)
+- [Lowest Common Ancestor of BST](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/)
+
+## Trie
+
+- Tries are special trees (prefix trees) that make searching and storing strings more efficient. Tries have many practical applications such as for searching and autocomplete. It will be helpful to know these common applications so that you can easily identify when a problem can be solved efficiently using a trie.
+- Sometimes preprocessing a dictionary of words (given in a list) into a trie will improve the efficiency when searching for a word of length k among n words. Searching becomes O(k) instead of O(n).
+- LeetCode has written a [very comprehensive article](https://leetcode.com/articles/implement-trie-prefix-tree/) on tries which you are highly encouraged to read.
+- Be familiar with implementing a `Trie` class and its `add`, `remove` and `search` methods from scratch.
+
+#### Practice Questions
+
+- [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree)
+- [Add and Search Word](https://leetcode.com/problems/add-and-search-word-data-structure-design)
+- [Word Search II](https://leetcode.com/problems/word-search-ii/)
+
+## Heap
+
+- If you see a top/lowest K being mentioned in the question, it is usually a signal that a heap can be used to solve the problem.
+- If you require the top K elements use a Min Heap of size K. Iterate through each element, pushing it into the heap. Whenever the heap size exceeds K, remove the minimum element, that will guarantee that you have the K largest elements.
+
+#### Practice Questions
+
+- [Merge K Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)
+- [Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/)
+- [Find Median from Data Stream](https://leetcode.com/problems/find-median-from-data-stream/)
+
+###### References
+
+- http://blog.triplebyte.com/how-to-pass-a-programming-interview
+- https://quip.com/q41AA3OmoZbC
+- http://www.geeksforgeeks.org/must-do-coding-questions-for-companies-like-amazon-microsoft-adobe/

algorithms/array.md

@@ -0,0 +1,63 @@
+Arrays
+==
+
+- In an arrays of arrays, e.g. given `[[], [1, 2, 3], [4, 5], [], [], [6, 7], [8], [9, 10], [], []]`, print: `1, 2, 3, 4, 5, 6, 7, 8, 9, 10`.
+  - Implement an iterator that supports `hasNext()`, `next()` and `remove()` methods.
+- Given a list of item prices, find all possible combinations of items that sum a particular value `K`.
+- Paginate an array with constraints, such as skipping certain items.
+- Implement a circular buffer using an array.
+- Given array of arrays, sort them in ascending order.
+- Given an array of integers, print out a histogram of using said array; include a base layer (all stars)
+  - E.g. `[5, 4, 0, 3, 4, 1]`
+
+```
+*
+**  *
+** **
+** **
+** ***
+******
+```
+
+- Given an array and an index, find the product of the elements of the array except the element at that index.
+- Given a set of rectangles represented by a height and an interval along the y-axis, determine the size of its union.
+- Given 2 separate arrays, write a method to find the values that exist in both arrays and return them.
+- Given an array of integers find whether there is a sub-sequence that sums to 0 and return it.
+  - E.g. `[1, 2, -3, 1]` => `[1, 2, -3]` or `[2, -3, 1]`.
+- Given an input array and another array that describes a new index for each element, mutate the input array so that each element ends up in their new index. Discuss the runtime of the algorithm and how you can be sure there would not be any infinite loops.
+- Given an array of non-negative numbers, find continuous subarray with sum to S.
+  - [Source](http://blog.gainlo.co/index.php/2016/06/01/subarray-with-given-sum/).
+- Given an array of numbers list out all triplets that sum to 0. Do so with a running time of less than O(n^3).
+  - [Source](http://blog.gainlo.co/index.php/2016/07/19/3sum/).
+- Given an array of numbers list out all quadruplets that sum to 0. Do so with a running time of less than O(n^4).
+- Given an array of integers, move all the zeroes to the end while preserving the order of the other elements. You have to do it in-place and are not allowed to use any extra storage.
+- Given a list of people, and a function `knows(a, b)` that returns `true`/`false` if person `a` knows the person `b`. Write a function that finds a VIP, which everybody knows and he doesn't know anybody else.
+- Given an array of integers, find the subarray with the largest sum. Can you do it in linear time.
+  - Maximum subarray sum problem.
+- You have an array with the heights of an island (at point 1, point 2 etc) and you want to know how much water would remain on this island (without flowing away).
+  - Trapping rain water question.
+- Given a sequence of tasks like `A, B, C` (means 3 different tasks), and a cold time, which means you need to wait for that much time to start the next same task, output the best task-finishing sequence.
+  - E.g. input: `AAABBB, 2`, output: `AB_AB_AB` ( `_` represents do nothing and wait)
+- You are given a list of dominoes. Determine if any two of those dominoes add up to `[6, 6]`.
+  - E.g. `[1, 4]` + `[5, 2]`).
+- Given an array containing only digits `0-9`, add one to the number and return the array.
+  - E.g. Given `[1, 4, 2, 1]` which represents `1421`, return `[1, 4, 2, 2]` which represents `1422`.
+- Find the second maximum value in an array.
+- Given an array, find the longest arithmetic progression.
+- Rotate an array by an offset of k.
+- Remove duplicates in an unsorted array where the duplicates are at a distance of k or less from each other.
+- Given an unsorted list of integers, return true if the list contains any duplicates within k indices of each element. Do it faster than O(n^2).
+- Given an unsorted list of integers, return true if the list contains any fuzzy duplicates within k indices of each element. A fuzzy duplicate is another integer within d of the original integer. Do it faster than O(n^2).
+  - E.g. If d = 4, then 6 is a fuzzy duplicate of 3 but 8 is not.
+- Say you have an unordered list of numbers ranging from 1 to n, and one of the numbers is removed, how do you find that number? What if two numbers are removed?
+- Given an array of string, find the duplicated elements.
+  - [Source](http://blog.gainlo.co/index.php/2016/05/10/duplicate-elements-of-an-array/).
+- Given an array of integers, find a maximum sum of non-adjacent elements.
+  - E.g. `[1, 0, 3, 9, 2]` should return `10 (1 + 9)`.
+  - [Source](http://blog.gainlo.co/index.php/2016/12/02/uber-interview-question-maximum-sum-non-adjacent-elements/)
+- Given an array of integers, modify the array by moving all the zeros to the end (right side). The order of other elements doesn’t matter.
+  - E.g. `[1, 2, 0, 3, 0, 1, 2]`, the program can output `[1, 2, 3, 1, 2, 0, 0]`.
+  - [Source](http://blog.gainlo.co/index.php/2016/11/18/uber-interview-question-move-zeroes/).
+- Given an array, return the length of the longest increasing contiguous subarray.
+  - E.g., `[1, 3, 2, 3, 4, 8, 7, 9]`, should return `4` because the longest increasing array is `[2, 3, 4, 8]`.
+  - [Source](http://blog.gainlo.co/index.php/2017/02/02/uber-interview-questions-longest-increasing-subarray/).

algorithms/dynamic-programming.md

@@ -0,0 +1,27 @@
+Dynamic Programming
+==
+
+- Given a flight itinerary consisting of starting city, destination city, and ticket price (2D list) - find the optimal price flight path to get from start to destination. (A variation of Dynamic Programming Shortest Path)
+- Given some coin denominations and a target value `M`, return the coins combination with the minimum number of coins.
+  - Time complexity: `O(MN)`, where N is the number of distinct type of coins.
+  - Space complexity: `O(M)`.
+- Given a set of numbers in an array which represent number of consecutive days of AirBnb reservation requested, as a host, pick the sequence which maximizes the number of days of occupancy, at the same time, leaving at least a 1 day gap in-between bookings for cleaning.
+  - Problem reduces to finding the maximum sum of non-consecutive array elements.
+  - E.g.
+    ~~~
+    // [5, 1, 1, 5] => 10
+    The above array would represent an example booking period as follows -
+    // Dec 1 - 5
+    // Dec 5 - 6
+    // Dec 6 - 7
+    // Dec 7 - 12
+
+    The answer would be to pick Dec 1-5 (5 days) and then pick Dec 7-12 for a total of 10 days of occupancy, at the same time, leaving at least 1 day gap for cleaning between reservations.
+
+    Similarly,
+    // [3, 6, 4] => 7
+    // [4, 10, 3, 1, 5] => 15
+    ~~~
+- How many string representations are there for an integer where `a->1, b->2, ... z->26`.
+  - E.g. `26 => 2`. Because `26` can be encoded as `"z"` and `"bf"`.
+- Given a list of denominations (e.g., `[1, 2, 5]` means you have coins worth $1, $2, and $5) and a target number `k`, find all possible combinations, if any, of coins in the given denominations that add up to `k`. You can use coins of the same denomination more than once.

algorithms/geometry.md

@@ -0,0 +1,7 @@
+Geometry
+==
+
+- You have a plane with lots of rectangles on it, find out how many of them intersect.
+- Which data structure would you use to query the k-nearest points of a set on a 2D plane?
+- Given many points, find k points that are closest to the origin.
+- How would you triangulate a polygon?

algorithms/graph.md

@@ -0,0 +1,12 @@
+Graph
+==
+
+- Given a list of sorted words from an alien dictionary, find the order of the alphabet.
+  - Alien Dictionary Topological Sort question.
+- Find if a given string matches any path in a labeled graph. A path may contain cycles.
+- Given a friendship graph (an undirected graph where nodes represent people and edges means two people know each other), find all your 2nd degree connections (friends’ friends). Output these 2nd degree connections ranked by number of common friends (i.e 1st degree connections) with you, (example: if 2nd degree connection A has 10 common friends (1st degree connections) with you but 2nd degree connection B has 8 common friends (1st degree connections) with you, then A should be ranked first).
+- Given a bipartite graph, separate the vertices into two sets.
+- Given a list of email addresses, and a similarity function which says whether two email addresses are similar, write a function to separate the list into sets of email addresses that are similar to each other.
+- You are a thief trying to sneak across a rectangular field. There are alarms placed on the fields and they each have a circular sensing radius which will trigger if anyone steps into it. Each alarm may not have the same radius. Determine if you can get from one end of the field to the other end.
+- Given a graph and two nodes, determine if there exists a path between them.
+- Determine if a cycle exists in the graph.

algorithms/hash-table.md

@@ -0,0 +1,7 @@
+Hash Table
+==
+
+- Describe an implementation of a least-used cache, and big-O notation of it.
+- A question involving an API's integration with hash map where the buckets of hash map are made up of linked lists.
+- Implement data structure `Map` storing pairs of integers (key, value) and define following member functions in O(1) runtime: `void insert(key, value)`, `void delete(key)`, `int get(key)`, `int getRandomKey()`.
+  - [Source](http://blog.gainlo.co/index.php/2016/08/14/uber-interview-question-map-implementation/).

algorithms/heap.md

@@ -0,0 +1,5 @@
+Heap
+==
+
+- Merge `K` sorted lists together into a single list.
+- Given a stream of integers, write an efficient function that returns the median value of the integers.

algorithms/interval.md

@@ -0,0 +1,28 @@
+Interval
+==
+
+- Given a list of schedules, provide a list of times that are available for a meeting.
+  ```
+  [
+    [[4,5], [6,10], [12,14]],
+    [[4,5], [5,9], [13,16]],
+    [[11,14]]
+  ]
+
+  Example Output:
+  [[0,4], [11,12], [16,23]]
+  ```
+- You have a number of meetings (with their start and end times). You need to schedule them using the minimum number of rooms. Return the list of meetings in every room.
+- Interval ranges:
+  - Given 2 interval ranges, create a function to tell me if these ranges intersect. Both start and end are inclusive: `[start, end]`
+    - E.g. `[1, 4]` and `[5, 6]` => `false`
+    - E.g. `[1, 4]` and `[3, 6]` => `true`
+  - Given 2 interval ranges that intersect, now create a function to merge the 2 ranges into a single continuous range.
+    - E.g. `[1, 4]` and `[3, 6]` => `[1, 6]`
+  - Now create a function that takes a group of unsorted, unorganized intervals, merge any intervals that intersect and sort them. The result should be a group of sorted, non-intersecting intervals.
+  - Now create a function to merge a new interval into a group of sorted, non-intersecting intervals. After the merge, all intervals should remain
+  non-intersecting.
+- Given a list of meeting times, check if any of them overlap. The follow-up question is to return the minimum number of rooms required to accommodate all the meetings.
+  - [Source](http://blog.gainlo.co/index.php/2016/07/12/meeting-room-scheduling-problem/)
+- If you have a list of intervals, how would you merge them?
+  - E.g. `[1, 3], [8, 11], [2, 6]` => `[1, 6], [8-11]`

algorithms/linked-list.md

@@ -0,0 +1,13 @@
+Linked List
+==
+
+- Given a linked list, in addition to the next pointer, each node has a child pointer that can point to a separate list. With the head node, flatten the list to a single-level linked list.
+  - [Source](http://blog.gainlo.co/index.php/2016/06/12/flatten-a-linked-list/)
+- Reverse a singly linked list. Implement it recursively and iteratively.
+- Convert a binary tree to a doubly circular linked list.
+- Implement an LRU cache with O(1) runtime for all its operations.
+- Check distance between values in linked list.
+- A question involving an API's integration with hash map where the buckets of hash map are made up of linked lists.
+- Given a singly linked list (a list which can only be traversed in one direction), find the item that is located at 'k' items from the end. So if the list is a, b, c, d and k is 2 then the answer is 'c'. The solution should not search the list twice.
+- How can you tell if a Linked List is a Palindrome?
+- Implement a LRU cache with O(1) runtime for all its operations.

algorithms/math.md

@@ -0,0 +1,20 @@
+Math
+==
+
+- Create a square root function.
+- Given a string such as "123" or "67", write a function to output the number represented by the string without using casting.
+- Make a program that can print out the text form of numbers from 1 - 1000 (ex. 20 is "twenty", 105 is "one hundred and five").
+- Write a function that parses Roman numerals.
+  - E.g. `XIV` returns `14`.
+- Write in words for a given digit.
+  - E.g. `123` returns `one hundred and twenty three`.
+- Given a number `N`, find the largest number just smaller than `N` that can be formed using the same digits as `N`.
+- Compute the square root of `N` without using any existing functions.
+- Given numbers represented as binary strings, and return the string containing their sum.
+  - E.g. `add('10010', '101')` returns `'10111'`.
+- Take in an integer and return its english word-format.
+  - E.g. 1 -> "one", -10,203 -> "negative ten thousand two hundred and three".
+- Write a function that returns values randomly, according to their weight. Suppose we have 3 elements with their weights: A (1), B (1) and C (2). The function should return A with probability 25%, B with 25% and C with 50% based on the weights.
+  - [Source](http://blog.gainlo.co/index.php/2016/11/11/uber-interview-question-weighted-random-numbers/)
+- Given a number, how can you get the next greater number with the same set of digits?
+  - [Source](http://blog.gainlo.co/index.php/2017/01/20/arrange-given-numbers-to-form-the-biggest-number-possible/)

algorithms/matrix.md

@@ -0,0 +1,18 @@
+Matrix
+==
+
+- You're given a 3 x 3 board of a tile puzzle, with 8 tiles numbered 1 to 8, and an empty spot. You can move any tile adjacent to the empty spot, to the empty spot, creating an empty spot where the tile originally was. The goal is to find a series of moves that will solve the board, i.e. get `[[1, 2, 3], [4, 5, 6], [7, 8, - ]]` where - is the empty tile.
+- Boggle implementation. Given a dictionary, and a matrix of letters, find all the words in the matrix that are in the dictionary. You can go across, down or diagonally.
+- The values of the matrix will represent numbers of carrots available to the rabbit in each square of the garden. If the garden does not have an exact center, the rabbit should start in the square closest to the center with the highest carrot count. On a given turn, the rabbit will eat the carrots available on the square that it is on, and then move up, down, left, or right, choosing the the square that has the most carrots. If there are no carrots left on any of the adjacent squares, the rabbit will go to sleep. You may assume that the rabbit will never have to choose between two squares with the same number of carrots. Write a function which takes a garden matrix and returns the number of carrots the rabbit eats. You may assume the matrix is rectangular with at least 1 row and 1 column, and that it is populated with non-negative integers. For example,
+  - Example: `[[5, 7, 8, 6, 3], [0, 0, 7, 0, 4], [4, 6, 3, 4, 9], [3, 1, 0, 5, 8]]` should return `27`.
+- Print a matrix in a spiral fashion.
+- In the Game of life, calculate how to compute the next state of the board. Follow up was to do it if there were memory constraints (board represented by a 1 TB file).
+- Grid Illumination: Given an NxN grid with an array of lamp coordinates. Each lamp provides illumination to every square on their x axis, every square on their y axis, and every square that lies in their diagonal (think of a Queen in chess). Given an array of query coordinates, determine whether that point is illuminated or not. The catch is when checking a query all lamps adjacent to, or on, that query get turned off. The ranges for the variables/arrays were about: 10^3 < N < 10^9, 10^3 < lamps < 10^9, 10^3 < queries < 10^9.
+- You are given a matrix of integers. Modify the matrix such that if a row or column contains a 0, make the values in the entire row or column 0.
+- Given an N x N matrix filled randomly with different colors (no limit on what the colors are), find the total number of groups of each color - a group consists of adjacent cells of the same color touching each other.
+- You have a 4 x 4 board with characters. You need to write a function that finds if a certain word exists in the board. You can only jump to neighboring characters (including diagonally adjacent).
+- Count the number of islands in a binary matrix of 0's and 1's.
+- Check a 6 x 7 Connect 4 board for a winning condition.
+- Given a fully-filled Sudoku board, check whether fulfills the Sudoku condition.
+- Implement a function that checks if a player has won tic-tac-toe.
+- Given an N x N matrix of 1's and 0's, figure out if all of the 1's are connected.

algorithms/oop.md

@@ -0,0 +1,4 @@
+Object-Oriented Programming
+==
+
+- How would you design a chess game? What classes and objects would you use? What methods would they have?

algorithms/permutation.md

@@ -0,0 +1,12 @@
+Permutation
+==
+
+- You are given a 7 digit phone number, and you should find all possible letter combinations based on the digit-to-letter mapping on numeric pad and return only the ones that have valid match against a given dictionary of words.
+- Give all possible letter combinations from a phone number.
+- Generate all subsets of a string.
+- Print all possible `N` pairs of balanced parentheses.
+  - E.g. when `N` is `2`, the function should print `(())` and `()()`.
+  - E.g. when `N` is `3`, we should get `((()))`, `(()())`, `(())()`, `()(())`, `()()()`.
+  - [Source](http://blog.gainlo.co/index.php/2016/12/23/uber-interview-questions-permutations-parentheses/)
+- Given a list of arrays, return a list of arrays, where each array is a combination of one element in each given array.
+   - E.g. If the input is `[[1, 2, 3], [4], [5, 6]]`, the output should be `[[1, 4, 5], [1, 4, 6], [2, 4, 5], [2, 4, 6], [3, 4, 5], [3, 4, 6]]`.

algorithms/queue.md

@@ -0,0 +1,4 @@
+Queue
+==
+
+- Implement a Queue class from scratch with an existing bug, the bug is that it cannot take more than 5 elements.

algorithms/sorting-searching.md

@@ -0,0 +1,18 @@
+Sorting and Searching
+==
+
+- Sorting search results on a page given a certain set of criteria.
+- Sort a list of numbers in which each number is at a distance `K` from its actual position.
+- Given an array of integers, sort the array so that all odd indexes are greater than the even indexes.
+- Given users with locations in a list and a logged-in user with locations, find their travel buddies (people who shared more than half of your locations).
+- Search for an element in a sorted and rotated array.
+  - [Source](http://blog.gainlo.co/index.php/2017/01/12/rotated-array-binary-search/)
+- Sort a list where each element is no more than k positions away from its sorted position.
+- If you have `N` revisions of a program, write a program that will find and return the first bad revision given a `isBad(revision i)` function.
+- Search for an item in a sorted, but rotated, array.
+- Merge two sorted lists together.
+- Given a list of numbers and a function that returns Low, Medium, or High, sort the list by Lows, then Mediums, then Highs.
+- Give 3 distinct algorithms to find the K largest values in a list of N items.
+- Find the minimum element in a sorted rotated array in faster than O(n) time.
+- Write a function that takes a number as input and outputs the biggest number with the same set of digits.
+  - [Source](http://blog.gainlo.co/index.php/2017/01/20/arrange-given-numbers-to-form-the-biggest-number-possible/)

algorithms/stack.md

@@ -0,0 +1,8 @@
+Stack
+==
+
+- Implementation of an interpreter for a small language that does multiplication/addition/etc.
+- Design a `MinStack` data structure that supports a `min()` operation that returns the minimum value in the stack in O(1) time.
+- Write an algorithm to determine if all of the delimiters in an expression are matched and closed.
+  - E.g. `{ac[bb]}`, `[dklf(df(kl))d]{}` and `{[[[]]]}` are matched. But `{3234[fd` and `{df][d}` are not.
+  - [Source](http://blog.gainlo.co/index.php/2016/09/30/uber-interview-question-delimiter-matching/)

algorithms/string.md

@@ -0,0 +1,61 @@
+String
+==
+
+- Output list of strings representing a page of hostings given a list of CSV strings.
+- Given a list of words, find the word pairs that when concatenated form a palindrome.
+- Find the most efficient way to identify what character is out of place in a non-palindrome.
+- Implement a simple regex parser which, given a string and a pattern, returns a boolean indicating whether the input matches the pattern. By simple, we mean that the regex can only contain special character: `*` (star), `.` (dot), `+` (plus). The star means that there will be zero or more of previous character in that place in the pattern. The dot means any character for that position. The plus means one or more of previous character in that place in the pattern.
+- Find all words from a dictionary that are x edit distance away.
+- Given a string IP and number n, print all CIDR addresses that cover that range.
+- Write a function called `eval`, which takes a string and returns a boolean. This string is allowed 6 different characters: `0`, `1`, `&`, `|`, `(`, and `)`. `eval` should evaluate the string as a boolean expression, where `0` is `false`, `1` is `true`, `&` is an `and`, and `|` is an `or`.
+  - E.g `"(0 | (1 | 0)) & (1 & ((1 | 0) & 0))"`
+- Given a pattern string like `"abba"` and an input string like `"redbluebluered"`, return `true` if and only if there's a one to one mapping of letters in the pattern to substrings of the input.
+  - E.g. `"abba"` and `"redblueredblue"` should return `true`.
+  - E.g. `"aaaa"` and `"asdasdasdasd"` should return `true`.
+  - E.g. `"aabb"` and `"xyzabcxzyabc"` should return `false`.
+- If you received a file in chunks, calculate when you have the full file. Quite an open-ended question. Can assume chunks come with start and end, or size, etc.
+- Given a list of names (strings) and the width of a line, design an algorithm to display them using the minimum number of lines.
+- Design a spell-checking algorithm.
+- Count and say problem.
+- Longest substring with `K` unique characters.
+  - [Source](http://blog.gainlo.co/index.php/2016/04/12/find-the-longest-substring-with-k-unique-characters/)
+- Given a set of random strings, write a function that returns a set that groups all the anagrams together.
+  - [Source](http://blog.gainlo.co/index.php/2016/05/06/group-anagrams/)
+- Given a string, find the longest substring without repeating characters. For example, for string `'abccdefgh'`, the longest substring is `'cdefgh'`.
+  - [Source](http://blog.gainlo.co/index.php/2016/10/07/facebook-interview-longest-substring-without-repeating-characters/)
+- Given a string, return the string with duplicate characters removed.
+- How many string representations are there for an integer where `a->1, b->2, ... z->26`.
+  - E.g. `126` can be `'az'` or `'abf'`.
+- Write a function that receives a regular expression (allowed chars = from `'a'` to `'z'`, `'*'`, `'.'`) and a string containing lower case english alphabet characters and return `true` or `false` whether the string matches the regex.
+  - E.g. `'ab*a'`, `'abbbbba'` => `true`.
+  - E.g. `'ab*b.'`, `'aba'` => `true`.
+  - E.g. `'abc*'`, `'acccc'` => `false`.
+- Given a rectangular grid with letters, search if some word is in the grid.
+- Given two strings representing integer numbers (`'123'` , `'30'`) return a string representing the sum of the two numbers: `'153'`.
+- Given a really big file with a lots of Facebook posts, find the ten most-used words.
+- A professor wants to see if two students have cheated when writing a paper. Design a function `hasCheated(String s1, String s2, int N)` that evaluates to `true` if two strings have a common substring of length `N`.
+  - Follow up: Assume you don't have the possibility of using `String.contains()` and `String.substring()`. How would you implement this?
+- Print all permutations of a given string.
+- Parse a string containing numbers and `'+'`, `'-'` and parentheses. Evaluate the expression. `-2+(3-5)` should return `-4`.
+- Output a substring with at most `K` unique characters.
+  - E.g. `'aabc'` and `k` = 2 => `'aab'`.
+- Ensure that there are a minimum of `N` dashes between any two of the same characters of a string.
+  - E.g. `n = 2, string = 'ab-bcdecca'` => `'ab--bcdec--ca'`.
+- Find the longest palindrome in a string.
+- Give the count and the number following in the series.
+  - E.g. `1122344`, next: `21221324`, next: `12112211121214`.
+  - Count and say problem.
+- Compress a string by grouping consecutive similar questions together:
+  - E.g. `'aaabbbcc' => `'a3b3c2'`.
+- You have a string consisting of open and closed parentheses, but parentheses may be imbalanced. Make the parentheses balanced and return the new string.
+- Given a set of strings, return the smallest subset that contains prefixes for every string.
+  - E.g. `['foo', 'foog', 'food', 'asdf']` => `['foo', 'asdf']`.
+- Write a function that would return all the possible words generated when using a phone (pre-smartphone era) numpad to type.
+- Given a dictionary and a word, find the minimum number of deletions needed on the word in order to make it a valid word.
+  - [Source](http://blog.gainlo.co/index.php/2016/04/29/minimum-number-of-deletions-of-a-string/)
+- How to check if a string contains an anagram of another string?
+  - [Source](http://blog.gainlo.co/index.php/2016/04/08/if-a-string-contains-an-anagram-of-another-string/)
+- Find all k-lettered words from a string.
+- Given a string of open and close parentheses, find the minimum number of edits needed to balance a string of parentheses.
+- Run length encoding - Write a string compress function that returns `'R2G1B1'` given `'RRGB'`.
+- Write a function that finds all the different ways you can split up a word into a concatenation of two other words.

algorithms/topics.md

@@ -0,0 +1,89 @@
+Topics
+==
+
+## Arrays
+
+## Strings
+
+- Prefix trees (Tries)
+- Suffix trees
+- Suffix arrays
+- KMP
+- Rabin-Karp
+- Boyer-Moore
+
+## Sorting
+
+- Bubble sort
+- Insertion sort
+- Merge sort
+- Quick sort
+- Selection sort
+- Bucket sort
+- Radix sort
+- Counting sort
+
+## Linked Lists
+
+## Stacks
+
+## Queues
+
+## Hash tables
+
+- Collision resolution algorithms
+
+## Trees
+
+- BFS
+- DFS (inorder, postorder, preorder)
+- Height
+
+## Binary Search Trees
+
+- Insert node
+- Delete a node
+- Find element in BST
+- Find min, max element in BST
+- Get successor element in tree
+- Check if a binary tree is a BST or not
+
+## Heaps / Priority Queues
+
+- Insert
+- Bubble up
+- Extract max
+- Remove
+- Heapify
+- Heap sort
+
+## Graphs
+
+- Various implementations
+  - Adjacency matrix
+  - Adjacency list
+  - Adjacency map
+- Single-source shortest path
+- Dijkstra
+- Bellman-Ford
+- Topo sort
+- MST
+- Prim algorithm
+- Kruskal's algorithm
+- Union Find Data Structure
+- Count connected components in a graph
+- List strongly connected components in a graph
+- Check for bipartite graph
+
+## Dynamic Programming
+
+- Count Change
+- 0-1 Knapsack
+
+## System Design
+
+- http://www.hiredintech.com/system-design/
+- https://www.quora.com/How-do-I-prepare-to-answer-design-questions-in-a-technical-interview?redirected_qid=1500023
+- http://blog.gainlo.co/index.php/2015/10/22/8-things-you-need-to-know-before-system-design-interviews/
+- https://github.com/donnemartin/system-design-primer
+- https://github.com/jwasham/coding-interview-university/blob/master/extras/cheat%20sheets/system-design.pdf

algorithms/tree.md

@@ -0,0 +1,37 @@
+Tree
+==
+
+- Find the height of a tree.
+- Find the longest path from the root to leaf in a tree.
+- Find the deepest left leaf of a tree.
+- Print all paths of a binary tree.
+  - [Source](http://blog.gainlo.co/index.php/2016/04/15/print-all-paths-of-a-binary-tree/)
+- Second largest element of a BST.
+  - [Source](http://blog.gainlo.co/index.php/2016/06/03/second-largest-element-of-a-binary-search-tree/)
+- Given a binary tree and two nodes, how to find the common ancestor of the two nodes?
+  - [Source](http://blog.gainlo.co/index.php/2016/07/06/lowest-common-ancestor/)
+- Find the lowest common ancestor of two nodes in a binary search tree.
+- Print the nodes in an n-ary tree level by level, one printed line per level.
+- Given a directory of files and folders (and relevant functions), how would you parse through it to find equivalent files?
+- Write a basic file system and implement the commands ls, pwd, mkdir, create, rm, cd, cat, mv.
+- Compute the intersection of two binary search trees.
+- Given a tree, where the parent has any number of nodes and each node has a number, return the average of all the nodes on each level in an array.
+- Given a binary tree, output all the node to leaf paths of it.
+- Given a string of characters without spaces, is there a way to break the string into valid words without leftover characters?
+- Print a binary tree level by level.
+- Determine if a binary tree is "complete" (i.e, if all leaf nodes were either at the maximum depth or max depth-1, and were 'pressed' along the left side of the tree).
+- Find the longest path in a binary tree. The path may start and end at any node.
+- Determine if a binary tree is a BST.
+- Given a binary tree, serialize it into a string. Then deserialize it.
+- Print a binary tree by column.
+- Given a node, find the next element in a BST.
+- Find the shortest subtree that consist of all the deepest nodes. The tree is not binary.
+- Print out the sum of each row in a binary tree.
+- Pretty print a JSON object.
+- Convert a binary tree to a doubly circular linked list.
+- Find the second largest number in a binary tree.
+- Given a tree, find the longest branch.
+- Convert a tree to a linked list.
+- Given two trees, write code to find out if tree A is a subtree of tree B.
+- Deepest node in a tree.
+  - [Source](http://blog.gainlo.co/index.php/2016/04/26/deepest-node-in-a-tree/)

design/README.md

@@ -0,0 +1,94 @@
+Design Questions
+==
+
+## Guides
+
+- https://github.com/donnemartin/system-design-primer
+- https://github.com/checkcheckzz/system-design-interview
+- https://github.com/shashank88/system_design
+- https://gist.github.com/vasanthk/485d1c25737e8e72759f
+- http://www.puncsky.com/blog/2016/02/14/crack-the-system-design-interview/
+- https://www.palantir.com/2011/10/how-to-rock-a-systems-design-interview/
+- http://blog.gainlo.co/index.php/2017/04/13/system-design-interviews-part-ii-complete-guide-google-interview-preparation/
+
+## Flow
+
+#### A. Understand the problem and scope
+
+- Define the use cases, with interviewer's help.
+- Suggest additional features.
+- Remove items that interviewer deems out of scope.
+- Assume high availability is required, add as a use case.
+
+#### B. Think about constraints
+
+- Ask how many requests per month.
+- Ask how many requests per second (they may volunteer it or make you do the math).
+- Estimate reads vs. writes percentage.
+- Keep 80/20 rule in mind when estimating.
+- How much data written per second.
+- Total storage required over 5 years.
+- How much data reads per second.
+
+#### C. Abstract design
+
+- Layers (service, data, caching).
+- Infrastructure: load balancing, messaging.
+- Rough overview of any key algorithm that drives the service.
+- Consider bottlenecks and determine solutions.
+
+Source: https://github.com/jwasham/coding-interview-university#system-design-scalability-data-handling
+
+## Grading Rubrics
+
+- Problem Solving - How systematic is your approach to solving the problem step-by-step? Break down a problem into its core components.
+- Communication - How well do you explain your idea and communicate it with others?
+- Evaluation - How do you evaluate your system? Are you aware of the trade-offs made? How can you optimize it?
+- Estimation - How fast does your system need to be? How much space does it need? How much load will it experience?
+
+## Specific Topics
+
+- URL Shortener
+  - http://stackoverflow.com/questions/742013/how-to-code-a-url-shortener
+  - http://blog.gainlo.co/index.php/2016/03/08/system-design-interview-question-create-tinyurl-system/
+  - https://www.interviewcake.com/question/python/url-shortener
+- Collaborative Editor
+  - http://blog.gainlo.co/index.php/2016/03/22/system-design-interview-question-how-to-design-google-docs/
+- Photo Sharing App
+  - http://blog.gainlo.co/index.php/2016/03/01/system-design-interview-question-create-a-photo-sharing-app/
+- Social Network Feed
+  - http://blog.gainlo.co/index.php/2016/02/17/system-design-interview-question-how-to-design-twitter-part-1/
+  - http://blog.gainlo.co/index.php/2016/02/24/system-design-interview-question-how-to-design-twitter-part-2/
+  - http://blog.gainlo.co/index.php/2016/03/29/design-news-feed-system-part-1-system-design-interview-questions/
+- Trending Algorithm
+  - http://blog.gainlo.co/index.php/2016/05/03/how-to-design-a-trending-algorithm-for-twitter/
+- Facebook Chat
+  - http://blog.gainlo.co/index.php/2016/04/19/design-facebook-chat-function/
+- Key Value Store
+  - http://blog.gainlo.co/index.php/2016/06/14/design-a-key-value-store-part-i/
+  - http://blog.gainlo.co/index.php/2016/06/21/design-key-value-store-part-ii/
+- Recommendation System
+  - http://blog.gainlo.co/index.php/2016/05/24/design-a-recommendation-system/
+- Cache System
+  - http://blog.gainlo.co/index.php/2016/05/17/design-a-cache-system/
+- E-commerce Website
+  - http://blog.gainlo.co/index.php/2016/08/22/design-ecommerce-website-part/
+  - http://blog.gainlo.co/index.php/2016/08/28/design-ecommerce-website-part-ii/
+- Web Crawler
+  - http://blog.gainlo.co/index.php/2016/06/29/build-web-crawler/
+  - http://www.makeuseof.com/tag/how-do-search-engines-work-makeuseof-explains/
+  - https://www.quora.com/How-can-I-build-a-web-crawler-from-scratch/answer/Chris-Heller
+- YouTube
+  - http://blog.gainlo.co/index.php/2016/10/22/design-youtube-part/
+  - http://blog.gainlo.co/index.php/2016/11/04/design-youtube-part-ii/
+- Hit Counter
+  - http://blog.gainlo.co/index.php/2016/09/12/dropbox-interview-design-hit-counter/
+- Facebook Graph Search
+- Design [Lyft Line](https://www.lyft.com/line).
+- Design a promo code system (with same promo code, randomly generated promo code, and promo code with conditions).
+- Model a university.
+- How would you implement Pacman?
+- Sketch out an implementation of Asteroids.
+- Implement a spell checker.
+- Design the rubik cube.
+- Design a high-level interface to be used for card games (e.g. poker, blackjack etc).

design/collaborative-editor.md

@@ -0,0 +1,108 @@
+Collaborative Document Editor
+==
+
+## Variants
+
+- Design Google docs.
+- Design a collaborative code editor like Coderpad/Codepile.
+- Design a collaborative markdown editor.
+
+## Requirements Gathering
+
+- What is the intended platform?
+  - Web
+- What features are required?
+  - Creating a document
+  - Editing a document
+  - Sharing a document
+- Bonus features
+  - Document revisions and reverting
+  - Searching
+  - Commenting
+  - Chatting
+  - Executing code (in the case of code editor)
+- What is in a document?
+  - Text
+  - Images
+- Which metrics should we optimize for?
+  - Loading time
+  - Synchronization
+  - Throughput
+
+## Core Components
+
+- Front end
+  - WebSockets/long polling for real-time communication between front end and back end.
+- Back end services behind a reverse proxy.
+  - Reverse proxy will proxy the requests to the right server.
+  - Split into a few services for different purposes.
+  - The benefit of this is that each service can use different languages that best suits its purpose.
+- API servers for non-collaborative features and endpoints.
+  - Ruby/Rails/Django for the server that deals with CRUD operations on data models where performance is not that crucial.
+- WebSocket servers for handling document edits and publishing updates to listeners.
+  - Possibly Node/Golang for WebSocket server which will need high performance as updates are frequent.
+- Task queue to persist document updates to the database.
+- ELB in front of back end servers.
+- MySQL database.
+- S3 and CDN for images.
+
+## Data Modeling
+
+- What kind of database to use?
+  - Data is quite structured. Would go with SQL.
+- Design the necessary tables, its columns and its relations.
+  - `users`
+    - `id`
+    - `name`
+  - `document`
+    - `id`
+    - `owner_id`
+  - `permissions`
+    - `id`
+    - `name`
+  - `document_permissions`
+    - `id`
+    - `document_id`
+    - `user_id`
+
+## Collaborative Editing - Client
+
+- Upon loading of the page and document, the client should connect to the WebSocket server over the WebSocket protocol `ws://`.
+- Upon connection, perform a time sync with the server, possibly via Network Time Protocol (NTP).
+- The most straightforward way is to send the whole updated document content to the back end, and all users currently viewing the document will receive the updated document. However, there are a few problems with this approach:
+  - Race condition. If two users editing the document at the same time, the last one to edit will overwrite the changes by the previous user. One workaround is to lock the document when a user is currently editing it, but that will not make it real-time collaborative.
+  - A large payload (the whole document) is being sent to servers and published to users on each change, and the user is likely to already have most of the content. A lot of redundant data being sent.
+- A feasible approach would be to use operational transforms and send just the action deltas to the back end. The back end publishes the action deltas to the listeners. What is considered an action delta?
+  - (a) Changing a character/word, (b) inserting a character/word/image, (c) deleting a character/word.
+  - With this approach, the payload will contain only small amount of data, such as (a) type of change, (b) character/word, (c) position in document: line/column, (d) timestamp. Why is the timestamp needed? Read on to find out.
+- Updates can also be throttled and batched, to avoid flooding the web server with requests. For example, if a user inserts a
+
+## Back End
+
+The back end is split into a few portions: WebSocket server for receiving and broadcasting document updates, CRUD server for reading and writing non-document-related data, and a task queue for persistence of the document.
+
+## WebSocket Server
+
+- Languages and frameworks that support async requests and non-blocking I/O will be suitable for the collaborative editor server. Node and Golang comes to my mind.
+- However, the WebSocket server is not stateless, so is it not that straightforward to scale horizontally. One approach would be for a Load Balancer to use Redis to maintain a map of the client to the WebSocket server instance IP, such that subsequent requests from the same client will be routed to the same server.
+- Each document corresponds to a room (more of namespace). Users can subscribe to the events happening within a room.
+- When a action delta is being received, blast it out to the listeners within the room and add it to the task queue.
+
+## CRUD Server
+
+- Provides APIs for reading and writing non-document-related data, such as users, permissions.
+
+## Task Queue + Worker Service
+
+- Worker service retrieves messages from the task queue and writes the updated documents to the database in an async fashion.
+- Batch the actions together and perform one larger write that consists of multiple actions. For example, instead of persisting to the database once per addition of a word, combine these additions and write them into the database at once.
+- Publish the save completion event to the WebSocket server to be published to the listeners, informing that the latest version of the document is being saved.
+- Benefit of using a task queue is that as the amount of tasks in the queue goes up, we can scale up the number of worker services to clear the backlog of work faster.
+
+## Document Persistence
+
+TODO
+
+###### References
+
+- http://blog.gainlo.co/index.php/2016/03/22/system-design-interview-question-how-to-design-google-docs/

design/news-feed.md

@@ -0,0 +1,164 @@
+News Feed
+==
+
+## Variants
+
+- Design Facebook news feed.
+- Design Twitter news feed.
+- Design Quora feed.
+- Design Instagram feed.
+
+## Requirements Gathering
+
+- What is the intended platform?
+  - Mobile (mobile web or native)? Web? Desktop?
+- What features are required?
+  - CRUD posts.
+  - Commenting on posts.
+  - Sharing posts.
+  - Trending posts?
+  - Tag people?
+  - Hashtags?
+- What is in a news feed post?
+  - Author.
+  - Content.
+  - Media.
+  - Tags?
+  - Hashtags?
+  - Comments/Replies.
+  - Operations:
+    - CRUD
+    - Commenting/replying to a post.
+- What is in a news feed?
+  - Sequence of posts.
+  - Query pattern: query for a user's ranked news feed.
+  - Operations:
+    - Append - Fetch more posts.
+    - Delete - I don't want to see this.
+- Which metrics should we optimize for?
+  - User retention.
+  - Ads revenue.
+  - Fast loading time.
+  - Bandwidth.
+  - Server costs.
+
+## Core Components
+
+TODO
+
+## Data modeling
+
+- What kind of database to use?
+  - Data is quite structured. Would go with SQL.
+- Design the necessary tables, its columns and its relations.
+  - `users`
+  - `posts`
+  - `likes`
+  - `follows`
+  - `comments`
+
+> There are two basic objects: user and feed. For user object, we can store userID, name, registration date and so on so forth. And for feed object, there are feedId, feedType, content, metadata etc., which should support images and videos as well.
+>
+> If we are using a relational database, we also need to model two relations: user-feed relation and friend relation. The former is pretty straightforward. We can create a user-feed table that stores userID and corresponding feedID. For a single user, it can contain multiple entries if he has published many feeds.
+>
+> For friend relation, adjacency list is one of the most common approaches. If we see all the users as nodes in a giant graph, edges that connect nodes denote friend relation. We can use a friend table that contains two userIDs in each entry to model the edge (friend relation). By doing this, most operations are quite convenient like fetch all friends of a user, check if two people are friends.
+>
+> The system will first get all userIDs of friends from friend table. Then it fetches all feedIDs for each friend from user-feed table. Finally, feed content is fetched based on feedID from feed table. You can see that we need to perform 3 joins, which can affect performance.
+>
+> A common optimization is to store feed content together with feedID in user-feed table so that we don’t need to join the feed table any more. This approach is called denormalization, which means by adding redundant data, we can optimize the read performance (reducing the number of joins).
+>
+> The disadvantages are obvious:
+> - Data redundancy. We are storing redundant data, which occupies storage space (classic time-space trade-off).
+> - Data consistency. Whenever we update a feed, we need to update both feed table and user-feed table. Otherwise, there is data inconsistency. This increases the complexity of the system.
+> - Remember that there’s no one approach always better than the other (normalization vs denormalization). It’s a matter of whether you want to optimize for read or write.
+
+## Feed Display
+
+- The most straightforward way is to fetch posts from all the people you follow and render them sorted by time.
+- There can be many posts to fetch. How many posts should you fetch?
+ - What are the pagination approaches and the pros and cons of each approach?
+  - Offset by page size
+  - Offset by time
+- What data should the post contain when you initially fetch them?
+- Lazy loading approach for loading associated data: media, comments, people who liked the post.
+- Media
+  - If the post contains media such as images and videos, how should they be handled? Should they be loaded on the spot?
+  - A better way would be to fetch images only when they are about to enter the viewport.
+  - Videos should not autoplay. Only fetch the thumbnail for the video, and only play the video when user clicks play.
+  - If the content is being refetched, the media should be cached and not fetched over the wire again. This is especially important on mobile connections where data can be expensive.
+- Comments
+  - Should you fetch all the comments for a post? For posts by celebrities, they can contain a few hundred or thousand comments.
+  - Maybe fetch the top few comments and display them under the post, and the user is given the choice to "show all comments".
+- How does the user request for new content?
+  - Infinite scrolling.
+  - User has to tap next page.
+
+## Feed Ranking
+
+- First select features/signals that are relevant and then figure out how to combine them to calculate a final score.
+- How do you show the relevant posts that the user is interested in?
+  - Chronological - While a chronological approach works, it may not be the most engaging approach. For example, if a person posts 30 times within the last hour, his followers will have their news feed clogged up with his posts. Maybe set a cap on the number of time a person's posts can appear within the feed.
+  - Popularity - How many likes and comments does the post have? Does the user usually like posts by that person?
+- How do you determine which are the more important posts? A user might be more interested in a few-hour old post from a good friend than a very recent post from an acquaintance.
+- A common strategy is to calculate a post score based on various features and rank posts by its score.
+- Prior to 2013, Facebook was using the [EdgeRank](https://www.wikiwand.com/en/EdgeRank) algorithm to determine what articles should be displayed in a user's News Feed.
+- Edge Rank basically is using three signals: affinity score, edge weight and time decay.
+  - Affinity score (u) - For each news feed, affinity score evaluates how close you are with this user. For instance, you are more likely to care about feed from your close friends instead of someone you just met once.
+  - Edge weight (e) - Edge weight basically reflects importance of each edge. For instance, comments are worth more than likes.
+  - Time decay (d) - The older the story, the less likely users find it interesting.
+- Affinity score
+  - Various factors can be used to reflect how close two people are. First of all, explicit interactions like comment, like, tag, share, click etc. are strong signals we should use. Apparently, each type of interaction should have different weight. For instance, comments should be worth much more than likes.
+  - Secondly, we should also track the time factor. Perhaps you used to interact with a friend quite a lot, but less frequent recently. In this case, we should lower the affinity score. So for each interaction, we should also put the time decay factor.
+- A good ranking system can improve some core metrics - user retention, ads revenue, etc.
+
+## Feed Publishing
+
+TODO. Refer to http://blog.gainlo.co/index.php/2016/04/05/design-news-feed-system-part-2/.
+
+## Additional Features
+
+#### Tagging feature
+
+- Have a `tags` table that stores the relation between a post and the people tagged in it.
+
+#### Sharing feature
+
+- Add a column to `posts` table called `original_post_id`.
+- What should happen when the original post is deleted?
+  - The shared `posts` have to be deleted too.
+
+#### Notifications feature
+
+- When should notifications happen?
+- Can the user subscribe to only certain types of notifications?
+
+#### Trending feature
+
+- What constitutes trending? What signals would you look at? What weight would you give to each signal?
+- Most frequent hashtags over the last N hours.
+- Hottest search queries.
+- Fetch the recent most popular feeds and extract some common words or phrases.
+
+#### Search feature
+
+- How would you index the data?
+
+## Scalability
+
+- Master-slave replication.
+  - Write to master database and read from replica databases/in-memory data store.
+  - Post contents are being read more than they are updated. It is acceptable to have a slight lag between a user updating a post and followers seeing the updated content. Tweets are not even editable.
+- Data for real-time queries should be in memory, disk is for writes only.
+- Pre-computation offline.
+- Tracking number of likes and comments.
+  - Expensive to do a `COUNT` on the `likes` and `comments` for a post.
+  - Use Redis/Memcached for keeping track of how many likes/comments a post has. Increment when there's new activity, decrement when someone unlikes/deletes the comment.
+- Load balancer in front of your API servers.
+- Partitioning the data.
+
+###### References
+
+- [Design News Feed System (Part 1)](http://blog.gainlo.co/index.php/2016/03/29/design-news-feed-system-part-1-system-design-interview-questions/)
+- [Design News Feed System (Part 1)](http://blog.gainlo.co/index.php/2016/04/05/design-news-feed-system-part-2/)
+- [Etsy Activity Feeds Architecture](https://www.slideshare.net/danmckinley/etsy-activity-feeds-architecture)
+- [Big Data in Real-Time at Twitter](https://www.slideshare.net/nkallen/q-con-3770885)

design/search-engine.md

@@ -0,0 +1,6 @@
+Search Engine
+==
+
+###### References
+
+- [How Do Search Engines Work?](http://www.makeuseof.com/tag/how-do-search-engines-work-makeuseof-explains/)

domain/async-loading/index.html

@@ -0,0 +1,61 @@
+<!doctype html>
+<head>
+  <style>
+    body {
+      font-family: 'Helvetica', sans-serif;
+    }
+  </style>
+</head>
+<body>
+  <div>
+    <div id="chapters">
+    </div>
+  </div>
+  <script>
+    (() => {
+      const DOM = {
+        $chapters: document.getElementById('chapters'),
+      };
+      const URL = 'https://raw.githubusercontent.com/googlesamples/web-fundamentals/gh-pages/fundamentals/getting-started/primers';
+
+      function getJSON(url) {
+        return new Promise((resolve, reject) => {
+          const req = new XMLHttpRequest();
+          req.open('GET', url);
+          req.onload = () => {
+            if (req.status === 200) {
+              resolve(JSON.parse(req.response));
+              return;
+            }
+            reject(req.responseText);
+          }
+          req.onerror = (err) => {
+            reject(err);
+          }
+          req.send();
+        });
+      }
+
+      function init() {
+        getJSON(`${URL}/story.json`).then(story => {
+          const $heading = document.createRange().createContextualFragment(story.heading);
+          DOM.$chapters.before($heading);
+          return Promise.all(story.chapterUrls.map((url, index) => {
+            return getJSON(`${URL}/${url}`);
+          }));
+        }).then(chapters => {
+          const $chapters = document.createDocumentFragment();
+          chapters.forEach(chapter => {
+            $chapters.appendChild(document.createRange().createContextualFragment(chapter.html));
+          });
+          DOM.$chapters.appendChild($chapters);
+        }).catch(err => {
+          console.warn(err);
+        });
+      }
+
+      document.addEventListener('DOMContentLoaded', init);
+    })();
+  </script>
+</body>
+</html>

domain/databases.md

@@ -0,0 +1,8 @@
+Databases
+==
+
+## General
+
+- How should you store passwords in a database?
+  - http://www.geeksforgeeks.org/store-password-database/
+  - https://nakedsecurity.sophos.com/2013/11/20/serious-security-how-to-store-your-users-passwords-safely/

domain/networking.md

@@ -0,0 +1,6 @@
+Networking
+==
+
+- Given an IPv4 IP address p and an integer n, return a list of CIDR strings that most succinctly represents the range of IP addresses from p to (p + n).
+- Describe what happens when you enter a url in the web browser.
+- Define UDP/TCP and give an example of both.

domain/pagination-sorting/data.js

@@ -0,0 +1,677 @@
+const data = [
+  {
+    "_id": "591ad7e2d1b6119887420af5",
+    "age": 36,
+    "name": "Salas Mccarthy",
+    "gender": "male",
+    "company": "MAKINGWAY",
+    "email": "salasmccarthy@makingway.com",
+    "phone": "+1 (905) 546-3931"
+  },
+  {
+    "_id": "591ad7e2918b0ba1231b582d",
+    "age": 29,
+    "name": "Jodi Graham",
+    "gender": "female",
+    "company": "BITREX",
+    "email": "jodigraham@bitrex.com",
+    "phone": "+1 (861) 596-2691"
+  },
+  {
+    "_id": "591ad7e2bc19d181dcb64d57",
+    "age": 34,
+    "name": "Hollie Hardin",
+    "gender": "female",
+    "company": "ZENSURE",
+    "email": "holliehardin@zensure.com",
+    "phone": "+1 (822) 410-2433"
+  },
+  {
+    "_id": "591ad7e224d2fa7216132561",
+    "age": 26,
+    "name": "Bullock Cole",
+    "gender": "male",
+    "company": "ZILPHUR",
+    "email": "bullockcole@zilphur.com",
+    "phone": "+1 (839) 563-3350"
+  },
+  {
+    "_id": "591ad7e206c3fb3043ccf8a8",
+    "age": 30,
+    "name": "Peterson Mosley",
+    "gender": "male",
+    "company": "MOBILDATA",
+    "email": "petersonmosley@mobildata.com",
+    "phone": "+1 (974) 547-3461"
+  },
+  {
+    "_id": "591ad7e20a180de908b0960e",
+    "age": 34,
+    "name": "Lucille Jackson",
+    "gender": "female",
+    "company": "KINETICUT",
+    "email": "lucillejackson@kineticut.com",
+    "phone": "+1 (845) 443-3594"
+  },
+  {
+    "_id": "591ad7e2768b0cf4837ecf35",
+    "age": 40,
+    "name": "Cooper Leonard",
+    "gender": "male",
+    "company": "CHORIZON",
+    "email": "cooperleonard@chorizon.com",
+    "phone": "+1 (817) 418-2290"
+  },
+  {
+    "_id": "591ad7e270a640cf8f8a3e44",
+    "age": 27,
+    "name": "Iris Shepherd",
+    "gender": "female",
+    "company": "IDEALIS",
+    "email": "irisshepherd@idealis.com",
+    "phone": "+1 (858) 599-3628"
+  },
+  {
+    "_id": "591ad7e222ddd244aaba0315",
+    "age": 31,
+    "name": "Oneil Head",
+    "gender": "male",
+    "company": "LIMOZEN",
+    "email": "oneilhead@limozen.com",
+    "phone": "+1 (894) 440-3396"
+  },
+  {
+    "_id": "591ad7e2d36847b5e648587b",
+    "age": 28,
+    "name": "Swanson Singleton",
+    "gender": "male",
+    "company": "NSPIRE",
+    "email": "swansonsingleton@nspire.com",
+    "phone": "+1 (817) 510-2480"
+  },
+  {
+    "_id": "591ad7e2b78092f9fb866779",
+    "age": 37,
+    "name": "Sutton Odom",
+    "gender": "male",
+    "company": "GLUKGLUK",
+    "email": "suttonodom@glukgluk.com",
+    "phone": "+1 (961) 424-2812"
+  },
+  {
+    "_id": "591ad7e222736aa66a9b35cc",
+    "age": 31,
+    "name": "Chandler Kirk",
+    "gender": "male",
+    "company": "EWAVES",
+    "email": "chandlerkirk@ewaves.com",
+    "phone": "+1 (902) 524-3712"
+  },
+  {
+    "_id": "591ad7e2ac3f9abecd5175a7",
+    "age": 28,
+    "name": "Gentry Stanley",
+    "gender": "male",
+    "company": "GINK",
+    "email": "gentrystanley@gink.com",
+    "phone": "+1 (878) 592-2331"
+  },
+  {
+    "_id": "591ad7e2f79787dc605d61fb",
+    "age": 28,
+    "name": "Paulette Guthrie",
+    "gender": "female",
+    "company": "SEQUITUR",
+    "email": "pauletteguthrie@sequitur.com",
+    "phone": "+1 (820) 473-2926"
+  },
+  {
+    "_id": "591ad7e2ff61553783737bc0",
+    "age": 37,
+    "name": "Lesley Nash",
+    "gender": "female",
+    "company": "MELBACOR",
+    "email": "lesleynash@melbacor.com",
+    "phone": "+1 (849) 414-2115"
+  },
+  {
+    "_id": "591ad7e26f9c3fc95fa8e6f1",
+    "age": 22,
+    "name": "Frye Oneil",
+    "gender": "male",
+    "company": "OVERPLEX",
+    "email": "fryeoneil@overplex.com",
+    "phone": "+1 (999) 412-2403"
+  },
+  {
+    "_id": "591ad7e2eee8b7cd857a29d0",
+    "age": 40,
+    "name": "Christina Zamora",
+    "gender": "female",
+    "company": "REPETWIRE",
+    "email": "christinazamora@repetwire.com",
+    "phone": "+1 (860) 504-3885"
+  },
+  {
+    "_id": "591ad7e2be93544c81761053",
+    "age": 25,
+    "name": "Earlene Dunn",
+    "gender": "female",
+    "company": "PHARMEX",
+    "email": "earlenedunn@pharmex.com",
+    "phone": "+1 (831) 582-2974"
+  },
+  {
+    "_id": "591ad7e2cb672de927940ef4",
+    "age": 25,
+    "name": "Hoffman Pittman",
+    "gender": "male",
+    "company": "ENOMEN",
+    "email": "hoffmanpittman@enomen.com",
+    "phone": "+1 (882) 420-3186"
+  },
+  {
+    "_id": "591ad7e2aabaae50d75c21a3",
+    "age": 35,
+    "name": "Wilson Tran",
+    "gender": "male",
+    "company": "CAXT",
+    "email": "wilsontran@caxt.com",
+    "phone": "+1 (842) 529-3085"
+  },
+  {
+    "_id": "591ad7e2bdd6104be85bdd87",
+    "age": 22,
+    "name": "Hensley Hawkins",
+    "gender": "male",
+    "company": "LUDAK",
+    "email": "hensleyhawkins@ludak.com",
+    "phone": "+1 (902) 566-3308"
+  },
+  {
+    "_id": "591ad7e24796cbdc0eacb6c0",
+    "age": 36,
+    "name": "Gray Schultz",
+    "gender": "male",
+    "company": "EBIDCO",
+    "email": "grayschultz@ebidco.com",
+    "phone": "+1 (910) 532-2845"
+  },
+  {
+    "_id": "591ad7e204425cc3b121279a",
+    "age": 25,
+    "name": "Odonnell Livingston",
+    "gender": "male",
+    "company": "ANOCHA",
+    "email": "odonnelllivingston@anocha.com",
+    "phone": "+1 (991) 422-2754"
+  },
+  {
+    "_id": "591ad7e2997ac59b446d3a30",
+    "age": 36,
+    "name": "Shelton Lindsay",
+    "gender": "male",
+    "company": "DENTREX",
+    "email": "sheltonlindsay@dentrex.com",
+    "phone": "+1 (899) 567-2928"
+  },
+  {
+    "_id": "591ad7e2646dad2ff456b035",
+    "age": 40,
+    "name": "Mcclain Larson",
+    "gender": "male",
+    "company": "GENMOM",
+    "email": "mcclainlarson@genmom.com",
+    "phone": "+1 (934) 508-3477"
+  },
+  {
+    "_id": "591ad7e21e908e847ee6d3ea",
+    "age": 27,
+    "name": "Miranda Branch",
+    "gender": "female",
+    "company": "ROCKYARD",
+    "email": "mirandabranch@rockyard.com",
+    "phone": "+1 (989) 446-2387"
+  },
+  {
+    "_id": "591ad7e2d7905d2fcaa1ce18",
+    "age": 36,
+    "name": "Valencia Moreno",
+    "gender": "male",
+    "company": "KIOSK",
+    "email": "valenciamoreno@kiosk.com",
+    "phone": "+1 (807) 449-2626"
+  },
+  {
+    "_id": "591ad7e245e3c4ce18a92054",
+    "age": 40,
+    "name": "Witt Leblanc",
+    "gender": "male",
+    "company": "ENERSAVE",
+    "email": "wittleblanc@enersave.com",
+    "phone": "+1 (809) 491-3886"
+  },
+  {
+    "_id": "591ad7e27f2c70583d1cf34f",
+    "age": 32,
+    "name": "Glover Mccoy",
+    "gender": "male",
+    "company": "BYTREX",
+    "email": "glovermccoy@bytrex.com",
+    "phone": "+1 (928) 470-2426"
+  },
+  {
+    "_id": "591ad7e22768012595c933c9",
+    "age": 25,
+    "name": "Billie Dunlap",
+    "gender": "female",
+    "company": "INQUALA",
+    "email": "billiedunlap@inquala.com",
+    "phone": "+1 (903) 415-2136"
+  },
+  {
+    "_id": "591ad7e22df8d07559a29904",
+    "age": 21,
+    "name": "Bridget Hill",
+    "gender": "female",
+    "company": "TRIBALOG",
+    "email": "bridgethill@tribalog.com",
+    "phone": "+1 (954) 461-3390"
+  },
+  {
+    "_id": "591ad7e201b2bddbc81ae05e",
+    "age": 30,
+    "name": "Dawson Clements",
+    "gender": "male",
+    "company": "EURON",
+    "email": "dawsonclements@euron.com",
+    "phone": "+1 (926) 570-3444"
+  },
+  {
+    "_id": "591ad7e2c2aeb820e77c1d9a",
+    "age": 31,
+    "name": "Valdez Johnson",
+    "gender": "male",
+    "company": "RETRACK",
+    "email": "valdezjohnson@retrack.com",
+    "phone": "+1 (841) 506-2428"
+  },
+  {
+    "_id": "591ad7e2370ff047048bd688",
+    "age": 31,
+    "name": "Nichole Dickerson",
+    "gender": "female",
+    "company": "NEOCENT",
+    "email": "nicholedickerson@neocent.com",
+    "phone": "+1 (945) 437-3591"
+  },
+  {
+    "_id": "591ad7e2b77affba5a8f8603",
+    "age": 34,
+    "name": "Madden Weaver",
+    "gender": "male",
+    "company": "VERBUS",
+    "email": "maddenweaver@verbus.com",
+    "phone": "+1 (810) 598-3145"
+  },
+  {
+    "_id": "591ad7e21e692cc8247b1d83",
+    "age": 26,
+    "name": "Larsen Benson",
+    "gender": "male",
+    "company": "ISOSURE",
+    "email": "larsenbenson@isosure.com",
+    "phone": "+1 (890) 435-2219"
+  },
+  {
+    "_id": "591ad7e2de06284f2d2a8d19",
+    "age": 37,
+    "name": "Barnes Gaines",
+    "gender": "male",
+    "company": "RODEOCEAN",
+    "email": "barnesgaines@rodeocean.com",
+    "phone": "+1 (824) 484-2693"
+  },
+  {
+    "_id": "591ad7e20d9d8224dfbd292f",
+    "age": 28,
+    "name": "Pacheco Landry",
+    "gender": "male",
+    "company": "METROZ",
+    "email": "pachecolandry@metroz.com",
+    "phone": "+1 (813) 525-3821"
+  },
+  {
+    "_id": "591ad7e2c5348da6a981a148",
+    "age": 21,
+    "name": "Richard Alexander",
+    "gender": "male",
+    "company": "GADTRON",
+    "email": "richardalexander@gadtron.com",
+    "phone": "+1 (911) 513-3007"
+  },
+  {
+    "_id": "591ad7e2ca9ef31ba3b67d4f",
+    "age": 22,
+    "name": "Stone Campos",
+    "gender": "male",
+    "company": "TETAK",
+    "email": "stonecampos@tetak.com",
+    "phone": "+1 (879) 440-3672"
+  },
+  {
+    "_id": "591ad7e2ff5a9796c8470151",
+    "age": 31,
+    "name": "Walter Randall",
+    "gender": "male",
+    "company": "EQUITAX",
+    "email": "walterrandall@equitax.com",
+    "phone": "+1 (927) 563-3777"
+  },
+  {
+    "_id": "591ad7e272c6b6c1241b3a4a",
+    "age": 27,
+    "name": "Copeland Compton",
+    "gender": "male",
+    "company": "MAXIMIND",
+    "email": "copelandcompton@maximind.com",
+    "phone": "+1 (829) 599-2574"
+  },
+  {
+    "_id": "591ad7e2220c6caa9e2c444d",
+    "age": 30,
+    "name": "Lorie Irwin",
+    "gender": "female",
+    "company": "ELPRO",
+    "email": "lorieirwin@elpro.com",
+    "phone": "+1 (938) 584-2368"
+  },
+  {
+    "_id": "591ad7e2942d603032f1ebda",
+    "age": 35,
+    "name": "Buck Roth",
+    "gender": "male",
+    "company": "COMTRACT",
+    "email": "buckroth@comtract.com",
+    "phone": "+1 (830) 585-3168"
+  },
+  {
+    "_id": "591ad7e2c23efccd2fcd5b2e",
+    "age": 31,
+    "name": "Mamie Robinson",
+    "gender": "female",
+    "company": "ZOMBOID",
+    "email": "mamierobinson@zomboid.com",
+    "phone": "+1 (845) 481-3222"
+  },
+  {
+    "_id": "591ad7e285109fadbf0f9c01",
+    "age": 33,
+    "name": "Bryan Stone",
+    "gender": "male",
+    "company": "PODUNK",
+    "email": "bryanstone@podunk.com",
+    "phone": "+1 (971) 469-3832"
+  },
+  {
+    "_id": "591ad7e25abc328c2cc42283",
+    "age": 37,
+    "name": "Benson Lyons",
+    "gender": "male",
+    "company": "FUTURITY",
+    "email": "bensonlyons@futurity.com",
+    "phone": "+1 (964) 536-3522"
+  },
+  {
+    "_id": "591ad7e28fa81611cdf5066a",
+    "age": 37,
+    "name": "Sally Finley",
+    "gender": "female",
+    "company": "ZYTRAC",
+    "email": "sallyfinley@zytrac.com",
+    "phone": "+1 (953) 506-2428"
+  },
+  {
+    "_id": "591ad7e271c5882aa5452601",
+    "age": 26,
+    "name": "Callahan Mckee",
+    "gender": "male",
+    "company": "ACCUPRINT",
+    "email": "callahanmckee@accuprint.com",
+    "phone": "+1 (842) 400-3570"
+  },
+  {
+    "_id": "591ad7e2275e787c5d9eea26",
+    "age": 25,
+    "name": "Ines Hamilton",
+    "gender": "female",
+    "company": "PROSURE",
+    "email": "ineshamilton@prosure.com",
+    "phone": "+1 (976) 485-2663"
+  },
+  {
+    "_id": "591ad7e2d8790e386b671ec6",
+    "age": 38,
+    "name": "Mcknight Abbott",
+    "gender": "male",
+    "company": "MAXEMIA",
+    "email": "mcknightabbott@maxemia.com",
+    "phone": "+1 (960) 600-3920"
+  },
+  {
+    "_id": "591ad7e2a1cae1e4f17d8db2",
+    "age": 28,
+    "name": "Tanisha Ross",
+    "gender": "female",
+    "company": "SYNKGEN",
+    "email": "tanishaross@synkgen.com",
+    "phone": "+1 (920) 502-3204"
+  },
+  {
+    "_id": "591ad7e2f816fd6848bc03a3",
+    "age": 22,
+    "name": "Pam Webb",
+    "gender": "female",
+    "company": "VALREDA",
+    "email": "pamwebb@valreda.com",
+    "phone": "+1 (926) 442-2683"
+  },
+  {
+    "_id": "591ad7e208585adb332ac653",
+    "age": 29,
+    "name": "Bobbi Mays",
+    "gender": "female",
+    "company": "PARLEYNET",
+    "email": "bobbimays@parleynet.com",
+    "phone": "+1 (940) 577-3487"
+  },
+  {
+    "_id": "591ad7e2f4b29cbd1744b14e",
+    "age": 27,
+    "name": "Snider Sandoval",
+    "gender": "male",
+    "company": "GRONK",
+    "email": "snidersandoval@gronk.com",
+    "phone": "+1 (903) 526-2655"
+  },
+  {
+    "_id": "591ad7e23a3f54209ec9deba",
+    "age": 33,
+    "name": "Fitzpatrick Weiss",
+    "gender": "male",
+    "company": "MICROLUXE",
+    "email": "fitzpatrickweiss@microluxe.com",
+    "phone": "+1 (894) 494-2135"
+  },
+  {
+    "_id": "591ad7e229d21e1b7723c044",
+    "age": 27,
+    "name": "Kimberly Brown",
+    "gender": "female",
+    "company": "SPACEWAX",
+    "email": "kimberlybrown@spacewax.com",
+    "phone": "+1 (832) 481-2926"
+  },
+  {
+    "_id": "591ad7e247e285f0c563ac94",
+    "age": 20,
+    "name": "Fisher Kent",
+    "gender": "male",
+    "company": "ORBIN",
+    "email": "fisherkent@orbin.com",
+    "phone": "+1 (962) 523-3956"
+  },
+  {
+    "_id": "591ad7e274439c6eb52e99df",
+    "age": 26,
+    "name": "Arlene Carroll",
+    "gender": "female",
+    "company": "ELITA",
+    "email": "arlenecarroll@elita.com",
+    "phone": "+1 (998) 497-3751"
+  },
+  {
+    "_id": "591ad7e271852e5766fad809",
+    "age": 22,
+    "name": "Tamra Spence",
+    "gender": "female",
+    "company": "ARTWORLDS",
+    "email": "tamraspence@artworlds.com",
+    "phone": "+1 (963) 516-2492"
+  },
+  {
+    "_id": "591ad7e28c9165c4372744f7",
+    "age": 29,
+    "name": "Alice Goodman",
+    "gender": "female",
+    "company": "ZILLIDIUM",
+    "email": "alicegoodman@zillidium.com",
+    "phone": "+1 (829) 577-2972"
+  },
+  {
+    "_id": "591ad7e2c2a9121f713f15c3",
+    "age": 27,
+    "name": "Clay Washington",
+    "gender": "male",
+    "company": "EXOSTREAM",
+    "email": "claywashington@exostream.com",
+    "phone": "+1 (926) 460-3699"
+  },
+  {
+    "_id": "591ad7e2c99d41ce0347ac8f",
+    "age": 38,
+    "name": "Navarro Walsh",
+    "gender": "male",
+    "company": "ILLUMITY",
+    "email": "navarrowalsh@illumity.com",
+    "phone": "+1 (896) 564-3270"
+  },
+  {
+    "_id": "591ad7e25e7c6ff0785d1337",
+    "age": 22,
+    "name": "Alexandra Hughes",
+    "gender": "female",
+    "company": "TASMANIA",
+    "email": "alexandrahughes@tasmania.com",
+    "phone": "+1 (977) 509-3599"
+  },
+  {
+    "_id": "591ad7e2acb8a5785b6fd512",
+    "age": 40,
+    "name": "Vonda Oliver",
+    "gender": "female",
+    "company": "NEPTIDE",
+    "email": "vondaoliver@neptide.com",
+    "phone": "+1 (836) 595-2260"
+  },
+  {
+    "_id": "591ad7e20543a9fc7bc33515",
+    "age": 36,
+    "name": "Christa Nixon",
+    "gender": "female",
+    "company": "GLASSTEP",
+    "email": "christanixon@glasstep.com",
+    "phone": "+1 (966) 551-2149"
+  },
+  {
+    "_id": "591ad7e21b8e73ba4dbf7c9a",
+    "age": 34,
+    "name": "Weber Hurst",
+    "gender": "male",
+    "company": "SLUMBERIA",
+    "email": "weberhurst@slumberia.com",
+    "phone": "+1 (943) 516-3449"
+  },
+  {
+    "_id": "591ad7e282b9cb56357b42db",
+    "age": 27,
+    "name": "Rasmussen Kramer",
+    "gender": "male",
+    "company": "MUSIX",
+    "email": "rasmussenkramer@musix.com",
+    "phone": "+1 (891) 553-3264"
+  },
+  {
+    "_id": "591ad7e2287c587e3a04e80c",
+    "age": 30,
+    "name": "Mayra Beasley",
+    "gender": "female",
+    "company": "BRAINCLIP",
+    "email": "mayrabeasley@brainclip.com",
+    "phone": "+1 (958) 576-2642"
+  },
+  {
+    "_id": "591ad7e26245135989367fa8",
+    "age": 20,
+    "name": "Nunez Ortiz",
+    "gender": "male",
+    "company": "KYAGURU",
+    "email": "nunezortiz@kyaguru.com",
+    "phone": "+1 (906) 591-3563"
+  },
+  {
+    "_id": "591ad7e23248cee3b84bde40",
+    "age": 39,
+    "name": "Michael Foreman",
+    "gender": "male",
+    "company": "DANCITY",
+    "email": "michaelforeman@dancity.com",
+    "phone": "+1 (848) 400-3539"
+  },
+  {
+    "_id": "591ad7e23ae236a1d59d07ee",
+    "age": 36,
+    "name": "Virgie Wheeler",
+    "gender": "female",
+    "company": "XIIX",
+    "email": "virgiewheeler@xiix.com",
+    "phone": "+1 (831) 400-2814"
+  },
+  {
+    "_id": "591ad7e2d8b9e3c0c31d770d",
+    "age": 31,
+    "name": "Levy Hoover",
+    "gender": "male",
+    "company": "CANDECOR",
+    "email": "levyhoover@candecor.com",
+    "phone": "+1 (843) 428-2780"
+  },
+  {
+    "_id": "591ad7e2badb7200ca06a483",
+    "age": 21,
+    "name": "Battle Stanton",
+    "gender": "male",
+    "company": "CALCULA",
+    "email": "battlestanton@calcula.com",
+    "phone": "+1 (861) 522-3951"
+  },
+  {
+    "_id": "591ad7e27fd1a4455ccf406c",
+    "age": 24,
+    "name": "Church Lynn",
+    "gender": "male",
+    "company": "DAISU",
+    "email": "churchlynn@daisu.com",
+    "phone": "+1 (896) 425-3854"
+  }
+]

domain/pagination-sorting/index.html

@@ -0,0 +1,144 @@
+<!doctype html>
+<head>
+  <style>
+    body {
+      font-family: 'Helvetica', sans-serif;
+    }
+  </style>
+</head>
+<body>
+  <h1>People</h1>
+  <table class="js-table">
+    <tr class="js-table-header">
+      <th data-field="name">Name</th>
+      <th data-field="age">Age</th>
+      <th data-field="email">Email</th>
+    </tr>
+    <tbody class="js-table-body">
+    </tbody>
+  </table>
+  <button class="js-page-button button-prev" data-type="prev">Prev</button>
+  Page <span class="js-current-page"></span> / <span class="js-total-page"></span>
+  <button class="js-page-button button-next" data-type="next">Next</button>
+
+  <script type="text/javascript" src="data.js"></script>
+  <script>
+    (() => {
+      function init() {
+        const DOM = {
+          $pageButtons: document.querySelectorAll('.js-page-button'),
+          $prevButton: document.querySelector('.js-page-button.button-prev'),
+          $nextButton: document.querySelector('.js-page-button.button-next'),
+          $tableHeader: document.querySelector('.js-table-header'),
+          $tableBody: document.querySelector('.js-table-body'),
+          $currentPage: document.querySelector('.js-current-page'),
+          $totalPages: document.querySelector('.js-total-page'),
+        };
+
+        const PAGE_SIZE = 10;
+        function initialState() {
+          return {
+            currentPage: 0,
+            totalPages: 0,
+            sortField: 'name',
+            sortOrder: 'asc',
+            data,
+          };
+        }
+        let state = initialState();
+        state.totalPages = Math.ceil(state.data.length / PAGE_SIZE);
+
+        function navigatePages(type) {
+          let newCurrentPage = state.currentPage + (type === 'prev' ? -1 : 1);
+          newCurrentPage = Math.max(0, newCurrentPage);
+          newCurrentPage = Math.min(newCurrentPage, state.totalPages - 1);
+          state.currentPage = newCurrentPage;
+        }
+
+        function setSortField(field) {
+          state.currentPage = 0;
+          if (state.sortField !== field) {
+            state.sortField = field;
+            state.sortOrder = 'asc';
+          } else {
+            state.sortOrder = state.sortOrder === 'asc' ? 'desc' : 'asc';
+          }
+        }
+
+        function attachEventListeners() {
+          // Pagination.
+          DOM.$pageButtons.forEach(el => {
+            el.addEventListener('click', function () {
+              navigatePages(this.getAttribute('data-type'));
+              render();
+            });
+          });
+
+          // Sorting.
+          DOM.$tableHeader.addEventListener('click', function (event) {
+            const el = event.target;
+            const field = el.getAttribute('data-field');
+            if (el.tagName !== 'TH' || !field) {
+              return;
+            }
+            setSortField(field);
+            render();
+          });
+        }
+
+        function render() {
+          DOM.$tableBody.innerHTML = '';
+
+          // Sort data.
+          const sortField = state.sortField;
+          state.data.sort((a, b) => {
+            switch (sortField) {
+              case 'name':
+              case 'email':
+                return a[sortField] > b[sortField] ? 1 : -1;
+              case 'age':
+                return a[sortField] - b[sortField];
+            }
+          });
+          if (state.sortOrder === 'desc') {
+            state.data.reverse();
+          }
+
+          // Create table rows.
+          const pageData = state.data.slice(state.currentPage * PAGE_SIZE, state.currentPage * PAGE_SIZE + PAGE_SIZE);
+          const $tableRowsFragment = document.createDocumentFragment();
+          pageData.forEach(person => {
+            const $tableRow = document.createElement('tr');
+            ['name', 'age', 'email'].forEach(field => {
+              const $tableCell = document.createElement('td');
+              $tableCell.textContent = person[field];
+              $tableRow.appendChild($tableCell);
+            });
+            $tableRowsFragment.appendChild($tableRow);
+          });
+          DOM.$tableBody.appendChild($tableRowsFragment);
+
+          // Pagination buttons disabled states.
+          DOM.$currentPage.textContent = state.currentPage + 1;
+          DOM.$totalPages.textContent = state.totalPages;
+          if (state.currentPage === 0) {
+            DOM.$prevButton.setAttribute('disabled', true);
+          } else {
+            DOM.$prevButton.removeAttribute('disabled');
+          }
+          if (state.currentPage === state.totalPages - 1) {
+            DOM.$nextButton.setAttribute('disabled', true);
+          } else {
+            DOM.$nextButton.removeAttribute('disabled');
+          }
+        }
+
+        render();
+        attachEventListeners();
+      }
+
+      document.addEventListener('DOMContentLoaded', init);
+    })();
+  </script>
+</body>
+</html>

domain/security.md

@@ -0,0 +1,67 @@
+Security
+==
+
+## Encryption
+
+#### Symmetrical Encryption
+
+- Symmetrical encryption is a type of encryption where one key can be used to encrypt messages and also decrypt the same message.
+- Symmetrical encryption is usually much less computationally expensive as compared to asymmetric encryption.
+- Often called "shared secret" encryption, or "secret key" encryption.
+- To use a symmetric encryption scheme, the sender and receiver must securely share a key in advance. This sharing can be done via asymmetric encryption.
+
+#### Asymmetric Encryption
+
+- A pair of keys are required: a **private key** and a **public key**. Public keys can be shared with anyone while private keys should be kept secret and known only to the owner.
+- A private key can be used to decrypt a message encrypted by a public key. A successful decryption verifies that the holder possesses the private key.
+- Also known as public key cryptography.
+
+## Public Key Infrastructure
+
+A public key infrastructure (PKI) is a system for the creation, storage, and distribution of digital certificates which are used to verify that a particular public key belongs to a certain entity. The PKI creates digital certificates which map public keys to entities, securely stores these certificates in a central repository and revokes them if needed.
+
+###### References
+
+- https://www.wikiwand.com/en/Public_key_infrastructure
+
+## SSH
+
+An SSH session consists of two stages, **Negotiating Encryption** and **User Authentication**.
+
+#### Negotiating Encryption
+
+The goal of this stage is for the client and server to agree upon and establish encryption to protect future communication, by generating an identical session key. One possible algorithm to generate the key is the Diffie–Hellman key exchange scheme. Each party generates a public/private key pair and exchanges the public key. After obtaining an authentic copy of each other's public keys, each party can compute a shared secret offline.
+
+The basis of this procedure for classic Diffie-Hellman is:
+
+1. Both parties agree on a large prime number, which will serve as a seed value.
+1. Both parties agree on an encryption generator (typically AES), which will be used to manipulate the values in a predefined way.
+1. Independently, each party comes up with another prime number which is kept secret from the other party. This number is used as the private key for this interaction (different than the private SSH key used for authentication).
+1. The generated private key, the encryption generator, and the shared prime number are used to generate a public key that is derived from the private key, but which can be shared with the other party.
+1. Both participants then exchange their generated public keys.
+1. The receiving entity uses their own private key, the other party's public key, and the original shared prime number to compute a shared secret key.
+1. Although this is independently computed by each party, using opposite private and public keys, it will result in the same shared secret key.
+1. The shared secret is then used to encrypt all communication that follows.
+
+The purpose of the shared secret key is to wrap all further communication in an encrypted tunnel that cannot be deciphered by outsiders.
+
+#### User Authentication
+
+The goal of this stage is to authenticate the user and discover whether access to the server should be granted. There are two approaches for authenticating, either by using passwords, or SSH key pairs.
+
+For password authentication, the server simply prompts the client for the password of the account they are attempting to login with. The password is sent through the negotiated encryption, so it is secure from outside parties.
+
+Authentication using SSH key pairs begins after the symmetric encryption has been established as described in the last section. The procedure happens like this:
+
+1. The client begins by sending an ID for the key pair it would like to authenticate with to the server.
+1. The server check's the `authorized_keys` file of the account that the client is attempting to log into for the key ID.
+1. If a public key with matching ID is found in the file, the server generates a random number and uses the public key to encrypt the number.
+1. The server sends the client this encrypted message.
+1. If the client actually has the associated private key, it will be able to decrypt the message using that key, revealing the original number.
+1. The client combines the decrypted number with the shared session key that is being used to encrypt the communication, and calculates the MD5 hash of this value.
+1. The client then sends this MD5 hash back to the server as an answer to the encrypted number message.
+1. The server uses the same shared session key and the original number that it sent to the client to calculate the MD5 value on its own. It compares its own calculation to the one that the client sent back. If these two values match, it proves that the client was in possession of the private key and the client is authenticated.
+
+###### References
+
+- https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process

domain/snake-game/snake-game.md

@@ -0,0 +1,77 @@
+Snake Game
+==
+
+Design a snake game that is to be played in web browser.
+
+Client: React + Redux
+
+Rendering:
+Pixel-based graphics. Depending on the intended resolution, can divide the screen into N * M pixels. Can dynamically calculate the size of each pixel.
+
+Fruit: One pixel.
+Snake body: One pixel width made up of connected pixels.
+
+Model:
+{
+  fruit: {
+    x, y
+  },
+  snake: {
+    points: [(x, y), ...] # head is at index 0
+    direction: north/south/east/west
+  }
+  speed: 500,
+  points: 0
+}
+
+function update() {
+  next_loc = points[0] + (x, y) # Depends on the direction
+  if (snake.points.find(next_loc) > 0) {
+     // die
+  }
+  let pts = snake.points;
+  if (!isEqual(next_loc, fruit)) {
+    pts = points.removeLast();
+  } else {
+    generate_fruit();
+    points++;
+  }
+  snake.points = [next_loc, ...pts];
+
+  // Boundary checking ->  die
+}
+
+function generate_fruit() {
+  // Cannot generate on my own body.
+
+  // First approach: while on body, generate
+  let next_fruit_location = random_location();
+  while (snake.points.find(next_fruit_location) > 0) {
+      next_fruit_location = random_location();
+  }
+  fruit = next_fruit_location
+
+  // Second approach: brute force
+  for (let i = 0; i < rows; i++) {
+    for (let j = 0; j < cols; j++) {
+      let point = { x: i, y: j }
+      if (snake.points.find(next_fruit_location) === -1) {
+        fruit = point
+      }
+    }
+  }
+
+  // Third approach: brute force with random
+  const available_points = []
+  for (let i = 0; i < rows; i++) {
+    for (let j = 0; j < cols; j++) {
+      let point = { x: i, y: j }
+      if (snake.points.find(next_fruit_location) === -1) {
+        available_points.push(point);
+      }
+    }
+  }
+  fruit = _.sample(available_points);
+}
+
+setInterval(update, speed);

domain/software-engineering.md

@@ -0,0 +1,22 @@
+Software Engineering
+==
+
+## What is the difference between an interface and abstract class?
+
+**Abstract Class**
+
+- For an abstract class, a method must be declared as abstract. An abstract method doesn't have an implementation.
+- The Abstract methods can be declared with Access modifiers like public, internal, protected, etc. When implementing these methods in a subclass, you must define them with the same (or a less restricted) visibility.
+- Abstract classes can contain variables and concrete methods.
+- A class can Inherit only one Abstract class. Hence multiple inheritance is not possible for an Abstract class.
+- Abstract is object-oriented. It offers the basic data an 'object' should have and/or functions it should be able to do. It is concerned with the object's basic characteristics: what it has and what it can do. Hence objects which inherit from the same abstract class share the basic characteristics (generalization).
+- Abstract class establishes "is a" relation with concrete classes.
+
+**Interface**
+
+- For an interface, all the methods are abstract by default. So one cannot declare variables or concrete methods in interfaces.
+- All methods declared in an interface must be public.
+- Interfaces cannot contain variables and concrete methods except constants.
+- A class can implement many interfaces. Hence multiple interface inheritance is possible.
+- Interface is functionality-oriented. It defines functionalities an object should have. Regardless what object it is, as long as it can do these functionalities, which are defined in the interface, it's fine. It ignores everything else. An object/class can contain several (groups of) functionalities; hence it is possible for a class to implement multiple interfaces.
+- Interface provides "has a" capability for classes.

domain/tic-tac-toe/index.html

@@ -0,0 +1,178 @@
+<!doctype html>
+<head>
+  <style>
+    body {
+      font-family: 'Helvetica', sans-serif;
+    }
+
+    .board-cell {
+      border: 1px solid #666;
+      box-sizing: border-box;
+      display: inline-block;
+      font-size: 32px;
+      height: 100px;
+      line-height: 100px;
+      text-align: center;
+      width: 100px;
+    }
+
+    .board-cell .content {
+      display: inline-block;
+      vertical-align: middle;
+    }
+  </style>
+</head>
+<body>
+  <h1>Tic Tac Toe</h1>
+  <p>Current player turn: <span class="js-current-player"></span></p>
+  <div class="js-board"></div>
+  <button class="js-reset">Reset</button>
+  <script>
+    // We will spend the next 45 minutes building a single-page web app that implements a Tic-Tac-Toe game. jQuery has been included for you. We'll implement the following features in order:
+
+    // 1. Render a 3x3 board. You can hardcode some X and O values within the cell for starters.
+    // 2. Implement the add symbol functionality that adds a X or O into a cell whenever the player clicks on it.
+    // 3. Rotate between the players whenever a move is made. Update the current player display.
+    // 4. Check for end game conditions after each move and display the winner if any.
+    // horizontally, vertically, diagonally
+    // 5. After a winner has been determined, disable further moves on the board.
+    // 6. Add a button to reset the game state.
+    (() => {
+      function init() {
+        const DOM = {
+          $currentPlayer: document.querySelector('.js-current-player'),
+          $board: document.querySelector('.js-board'),
+          $resetButton: document.querySelector('.js-reset'),
+        };
+        const SIZE = 3;
+        function initialState() {
+          return {
+            boardModel: Array(SIZE).fill(null).map(_ => Array(SIZE).fill(null)),
+            players: ['X', 'O'],
+            currentPlayer: 0,
+            gameEnded: false,
+            turn: 0,
+          };
+        }
+        let state = initialState();
+
+        function renderBoard() {
+          DOM.$currentPlayer.textContent = state.players[state.currentPlayer];
+          // Assuming SIZE > 0.
+          DOM.$board.innerHTML = '';
+          for (let i = 0; i < SIZE; i++) {
+            const $row = document.createElement('div');
+            $row.classList.add('board-row');
+            for (let j = 0; j < SIZE; j++) {
+              const $cell = document.createElement('div');
+              $cell.classList.add('board-cell');
+              $cell.setAttribute('data-i', i);
+              $cell.setAttribute('data-j', j);
+              const $content  = document.createElement('span');
+              $content.classList.add('content');
+              $content.textContent = state.boardModel[i][j];
+              $cell.appendChild($content);
+              $row.appendChild($cell);
+            }
+            DOM.$board.appendChild($row);
+          }
+        }
+
+        function checkWinning(board, player) {
+          // Check horizontal.
+          for (let i = 0; i < SIZE; i++) {
+            if (board[i].every(cell => cell === player)) {
+              return true;
+            }
+          }
+
+          // Check vertical.
+          for (let j = 0; j < SIZE; j++) {
+            let verticalAllPlayer = true;
+            for (let i = 0; i < SIZE; i++) {
+              if (board[i][j] !== player) {
+                verticalAllPlayer = false;
+                break;
+              }
+            }
+            if (verticalAllPlayer) {
+              return verticalAllPlayer;
+            }
+          }
+
+          // Check diagonal South-East.
+          let diagonalAllPlayer = true;
+          for (let i = 0; i < SIZE; i++) {
+            if (board[i][i] !== player) {
+              diagonalAllPlayer = false;
+              break;
+            }
+          }
+          if (diagonalAllPlayer) {
+            return diagonalAllPlayer;
+          }
+
+          // Check diagonal North-East.
+          diagonalAllPlayer = true;
+          for (let i = SIZE - 1, j = 0; i >= 0; i--, j++) {
+            if (board[i][j] !== player) {
+              diagonalAllPlayer = false;
+              break;
+            }
+          }
+          if (diagonalAllPlayer) {
+            return diagonalAllPlayer;
+          }
+
+          return false;
+        }
+
+        function attachEventListeners() {
+          DOM.$board.addEventListener('click', (event) => {
+            if (state.gameEnded) {
+              return;
+            }
+            if (!event.target.classList.contains('board-cell')) {
+              return;
+            }
+            const $cell = event.target;
+            const i = parseInt($cell.getAttribute('data-i'), 10);
+            const j = parseInt($cell.getAttribute('data-j'), 10);
+            if (state.boardModel[i][j] !== null) {
+              alert('Cell has already been taken!');
+              return;
+            }
+            const player = state.players[state.currentPlayer];
+            state.boardModel[i][j] = player;
+            const winningMove = checkWinning(state.boardModel, player);
+            state.turn++;
+            if (!winningMove) {
+              state.currentPlayer = (state.currentPlayer + 1) % 2;
+              renderBoard();
+              if (state.turn === SIZE * SIZE) {
+                alert('It\'s a draw!');
+              }
+            } else {
+              renderBoard();
+              state.gameEnded = true;
+              alert(`Player ${player} wins!`);
+            }
+          });
+
+          DOM.$resetButton.addEventListener('click', () => {
+            if (confirm('Start a new game?')) {
+              state = initialState();
+              renderBoard();
+            }
+          });
+        }
+
+        renderBoard();
+        attachEventListeners();
+      }
+
+      document.addEventListener('DOMContentLoaded', init);
+    })();
+  </script>
+</body>
+</html>

front-end/accessibility.md

@@ -0,0 +1,201 @@
+Accessibility
+==
+
+## Glossary
+
+- **Accessibility** -
+- **WAI-ARIA** - Web Accessibility Initiative - Accessible Rich Internet Applications. Commonly shortened to ARIA.
+
+## What is Accessibility?
+
+Making sure that the content and the websites we create are usable to people with impairments or disabilities.
+
+## WebAIM Checklist
+
+The following is a checklist that contains recommendations for implementing HTML-related principles and techniques for those seeking WCAG 2.0 conformance (it is NOT the Web Content Accessibility Guidelines (WCAG) 2.0).
+
+- **Perceivable** - Web content is made available to the senses - sight, hearing, and/or touch.
+  - Text Alternatives: Provide text alternatives for any non-text content.
+  - Time-based Media: Provide alternatives for time-based media.
+  - Adaptable: Create content that can be presented in different ways (for example simpler layout) without losing information or structure.
+  - Distinguishable: Make it easier for users to see and hear content including separating foreground from background.
+- **Operable** - Interface forms, controls, and navigation are operable.
+  - Keyboard Accessible: Make all functionality available from a keyboard.
+  - Enough Time: Provide users enough time to read and use content.
+  - Seizures: Do not design content in a way that is known to cause seizures.
+  - Navigable: Provide ways to help users navigate, find content, and determine where they are.
+- **Understandable** - Content and interface are understandable.
+  - Readable: Make text content readable and understandable.
+  - Predictable: Make Web pages appear and operate in predictable ways.
+  - Input Assistance: Help users avoid and correct mistakes.
+- **Robust** - Content can be used reliably by a wide variety of user agents, including assistive technologies.
+  - Compatible: Maximize compatibility with current and future user agents, including assistive technologies.
+
+**Source:** http://webaim.org/standards/wcag/checklist
+
+## Focus
+
+- Making sure your application has a sensible tab order is important.
+- HTML forms and inputs are focusable and handle keyboard events by default.
+- Focus tab order relies on the DOM order in the HTML.
+- Be careful when using CSS when changing the order of elements on the screen, it can cause the order to be unintuitive and messy.
+- `tabindex` attribute:
+  - `-1`: Not in the natural tab order, but programatically focusable using JavaScript with `focus()` method. Useful for off-screen content which later appears on screen. Children elements are **NOT** pulled out of the tab order.
+  - `0`: In the natural tab order and can be programatically focused.
+  - `1` (bigger than 1): In the natural tab order but jumped in front of the tab order regardless of where it is in the DOM. It can be considered an anti-pattern.
+- Add focus behavior to interactive controls, like buttons, tabs, dropdowns, stuff that users will interactive with.
+- Use skip links to allow users to skip directly to the main content without having to tab through all the navigation.
+- `document.activeElement` is useful in tracking the current element that has focus on.
+
+## Semantics
+
+- Using proper labeling not only helps accessibility but it makes the element easier to target for all users!
+- Use `<label>` with `for` attributes for form elements.
+- Use `alt` attribute for `<img>` elements. Alt text must describe the image.
+- TODO
+
+## Navigating Content
+
+- MacOS comes built-in with VoiceOver. Press <kbd>CMD</kbd> + <kbd>F5</kbd> to activate.
+  - Activate Web Rotor with <kbd>Ctrl</kbd> + <kbd>Option</kbd> + <kbd>U</kbd>. Web Rotor displays landmarks, headings, links and more on the page and allows you to jump to them directly.
+- Heading weight should be decided by its importance on the page and not how big it should look, as the heading tag chosen affects the order the headings are listed on screen readers.
+- Use HTML5 semantic tags like `<main>`, `<nav>`, `<header>`, `<aside>`, `<article>`, `<section>`, `<footer>` to indicate landmarks on the page.
+
+## ARIA
+
+- Express semantics that HTML can't express on its own.
+- Accessibility tree = DOM + ARIA.
+- ARIA attributes
+  - Allow us to modify the accessibility tree before they are exposed to assistive technologies.
+  - DO NOT modify the element apperance.
+  - DO NOT modify element behaviour.
+  - DO NOT add focusability.
+  - DO NOT add keyboard event handling.
+- E.g. for custom checkboxes, adding ARIA attributes is not sufficient, you will need to write your own JavaScript to emulate the native behaviour to synchronize the ARIA attributes and values with the actual visual state, such as toggling using clicks and hitting spacebar. It's probably not worth it to reinvent the wheel by writing your own custom widgets that already exist in HTML5.
+- ARIA can add semantics to elements that have no native semantics, such as `<div>`. ARIA can also modify element semantics.
+- ARIA allows developers to create accessible widgets that do not exist in HTML5, such as a tree widget.
+- `aria-role` attributes tell assistive technologies that the element should follow that role's accessibility patterns. There are well-defined roles in the HTML spec. Do not define them on your own.
+  - `tabindex="0"` is usually added to it elements that have `role` added so that it can be focused.
+- Assistive labelling
+  - `aria-label` is useful for labelling buttons where the content is empty or contains only icons.
+  - `aria-labelledby` is similar to `<label>` elements, and can be used on any elements.
+    ```html
+    /* Normal label example */
+    <input type="radio" id="coffee-label">
+    <label for="coffee-label">Coffee</label>
+
+    /* aria-labelledby example */
+    <div role="radio" aria-labelledby="coffee-label"></div>
+    <span id="coffee-label">Coffee</span>
+    ```
+- ARIA Relationships
+  - ARIA relationship attributes create semantic relationships between elements on the page. The `aria-labelledby` attribute in the previous example indicates that the `<div>` is labelled by the element with that `id`.
+  - Possible relationship attributes include `aria-activedescendent`, `aria-describedby`, `aria-labelledby`, `aria-owns`, `aria-posinset` and `aria-setsize`.
+- With ARIA, you can expose only relevant parts of the page to accessibility tree. Elements can be hidden via:
+  - Setting `visibility`: `<button style="visibility: hidden">`.
+  - Setting `display`: `<button style="display: none">`.
+  - HTML5 `hidden` attribute: `<span hidden>`. This makes the element hidden to everyone.
+  - `aria-hidden` attribute: `<div aria-hidden="true">`. This makes the element hidden to screenreaders too. Note that `aria-hidden` attribute requires an explicit value of `true` or `false`.
+- Technique for screenreader-only text:
+  ```
+  .screenreader {
+    position: absolute;
+    left: -1000px;
+    width: 1px;
+    height: 1px;
+    overflow: hidden;
+  }
+  ```
+- `aria-live` attribute can be used to grab the assistive technology's attention to cause it to announce updates to the user. Practically, include `aria-live` attributes in the initial page load. The different `aria-live` values include:
+  - `off` (default) - Updates will not be presented unless the region is currently focused.
+  - `polite` - Assistive technologies should announce updates at the next graceful opportunity, such as at the end of speaking the current sentence on when the user pauses typing. Such as receiving new chat messages.
+  - `assertive` - Highest priority and assistive technologies should notify the user immediately. Examples include server status error alerts.
+- `aria-atomic` attribute indicates whether the entire region should be presented as a whole when communicating updates. Such as a date widget comprising of multiple `<input>` fields for day/month/year. When the user changes a field, the full contents of the widget will be read out. It takes in a `true` or `false` value.
+- `aria-relevant` attribute indicates what types of changes should be presented to the user.
+  - `additions` - Element nodes are added to the DOM within the live region.
+  - `removals` - Text or element nodes within the live region are removed from the DOM.
+  - `text` - Text is added to any DOM descendant nodes of the live region.
+  - `all` - Equivalent to the combination of all values, `additions removals text`.
+  - `additions text` (default) - Equivalent to the combination of values, `additions text`.
+- `aria-busy` attribute indicates the assistive technologies should ignore changes to the element, such as when things are loading, for example after a temporary connectivity loss. It takes in `true` or `false`. It takes in a `true` or `false` value.
+
+## Style
+
+#### Introduction
+
+- Ensure elements are styled to support the existing accessibility work, such as adding styles for `:focus` and the various ARIA states.
+- Flexible user interface that can handle being zoomed or scaled up, for users who have trouble reading smaller text.
+- Color choices and the importance of contrast, making sure we are not conveying information just with color alone.
+
+#### Focus
+
+- As much as possible, leave the default focus in place. Do not remove the `:focus` styling just because it does not fit into your design or looks odd! - A good technique is to use a similar styling as `:hover` for `:focus`.
+- Some CSS resets would kill off the focus styling, so it's important to be aware of them and get them back.
+
+#### Styling with ARIA
+
+Consider using ARIA attributes in your CSS selectors to reduce some noise in your CSS. For custom toggle buttons, instead of doing this,
+
+```html
+<div class="toggle pressed" role="button" tabindex="0" aria-pressed="true"></div> /* On */
+<div class="toggle" role="button" tabindex="0" aria-pressed="false"></div> /* Off */
+
+.toggle.pressed {
+  ...
+}
+```
+
+you can do this instead:
+
+```html
+<div class="toggle" role="button" tabindex="0" aria-pressed="true"></div> /* On */
+<div class="toggle" role="button" tabindex="0" aria-pressed="false"></div> /* Off */
+
+.toggle[aria-pressed="true"] {
+  ...
+}
+```
+
+which removes the need for toggling the `press` class on the element.
+
+#### Responsive Design
+
+Responsive design is also beneficial for accessibility when zooming the page transforms the page into the mobile layout instead.
+
+Use a meta viewport tag:
+
+```
+<meta name="viewport" content="width=device-width initial-scale="1">
+```
+
+`user-scalable=no` is an anti-pattern for accessibility.
+
+Use relative units like `%`, `em` and `rem`. The differences are as follows:
+
+- `%` - Relative to the containing block.
+- `em` - Relative to the `font-size` of the parent.
+- `rem` - Relative to the `font-size` of the root, which is the `<html>` element.
+
+Interactive interface elements such as buttons should be large enough, have enough spacing around itself so that they do not overlap with other interactive elements.
+
+#### Color and Contrast
+
+Contrast ratio is the ratio of luminance between the foreground color (such as text) and the background color. For text and images, aim for a large contrast ratio of 7:1 and for larger text (over 18 point or 14 point bold), aim for at least 4.5:1.
+
+Chrome Devtools has an Accessibility audit feature that can flag the contrast issues on your page.
+
+Color should not be used as the sole method of conveying content or distinguishing visual elements, such as only changing the `border-color` of `<input>` fields that have error to red. These changes will not be obvious/visible to people with color blindness. An error message below the field will be helpful.
+
+Some users might be using a High Contrast mode which allows a user to invert the background and foreground colors to read text better. Ensure that your page also looks fine on High Contrast mode which you can simulate with a [Chrome High Contrast extension](https://chrome.google.com/webstore/detail/high-contrast/djcfdncoelnlbldjfhinnjlhdjlikmph?hl=en).
+
+#### Assessing Impact of Accessibility Issues
+
+Fixing accessibility issues is like fixing bugs; it is best looked at through the lens of impact. How can you have the most impact on users with the least amount of effort?
+
+- How frequent is this piece of UI used? Is it part of a critical flow?
+- How badly does this accessibility issue affect your users?
+- How expensive is it going to cost to fix?
+
+###### References
+
+- https://www.udacity.com/course/web-accessibility--ud891

front-end/browser.md

@@ -0,0 +1,59 @@
+Browser
+==
+
+## Glossary
+
+- **BOM** - The Browser Object Model (BOM) is a browser-specific convention referring to all the objects exposed by the web browser. The `window` object is one of them.
+- **CSSOM** - CSS Object Model.
+- **DOM** - The Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML, and XML documents.
+- **Reflow** - When the changes affect document contents or structure, or element position, a reflow (or relayout) happens.
+- **Repaint** - When changing element styles which don't affect the element's position on a page (such as `background-color`, `border-color`, `visibility`), the browser just repaints the element again with the new styles applied (that means a "repaint" or "restyle" is happening).
+- **Composite** - TODO
+
+## Rendering
+
+High level flow of how browsers render a webpage:
+
+1. DOM
+  - The DOM (Document Object Model) is formed from the HTML that is received from a server.
+  - Characters -> Tokens -> Nodes -> DOM.
+  - DOM construction is incremental.
+  - CSS and JS are requested as the respective `<link>` and `<script>` tags are encountered.
+1. CSSOM
+  - Styles are loaded and parsed, forming the CSSOM (CSS Object Model).
+  - Characters -> Tokens -> Nodes -> CSSOM.
+  - CSSOM construction is not incremental.
+  - Browser blocks page rendering until it receives and processes all the CSS.
+  - CSS is render blocking.
+1. Render Tree
+  - On top of DOM and CSSOM, a render tree is created, which is a set of objects to be rendered. Render tree reflects the DOM structure except for invisible elements (like the <head> tag or elements that have `display: none`; set). Each text string is represented in the rendering tree as a separate renderer. Each of the rendering objects contains its corresponding DOM object (or a text block) plus the calculated styles. In other words, the render tree describes the visual representation of a DOM.
+1. Layout
+  - For each render tree element, its coordinates are calculated, which is called "layout". Browsers use a flow method which only required one pass to layout all the elements (tables require more than one pass).
+1. Painting
+  - Finally, this gets actually displayed in a browser window, a process called "painting".
+
+###### References
+
+- https://www.html5rocks.com/en/tutorials/internals/howbrowserswork/webkitflow.png
+- https://medium.freecodecamp.org/its-not-dark-magic-pulling-back-the-curtains-from-your-stylesheets-c8d677fa21b2
+
+## Repaint
+
+When changing element styles which don't affect the element's position on a page (such as `background-color`, `border-color`, `visibility`), the browser just repaints the element again with the new styles applied (that means a "repaint" or "restyle" is happening).
+
+## Reflow
+
+When the changes affect document contents or structure, or element position, a reflow (or relayout) happens. These changes are usually triggered by:
+- DOM manipulation (element addition, deletion, altering, or changing element order)
+- Contents changes, including text changes in form fields
+- Calculation or altering of CSS properties
+- Adding or removing style sheets
+- Changing the "class" attribute
+- Browser window manipulation (resizing, scrolling); Pseudo-class activation (`:hover`)
+
+#### References
+
+- [How Browsers Work](https://www.html5rocks.com/en/tutorials/internals/howbrowserswork/)
+- [What Every Frontend Developer Should Know About Webpage Rendering](http://frontendbabel.info/articles/webpage-rendering-101/)
+- [Rendering: repaint, reflow/relayout, restyle](http://www.phpied.com/rendering-repaint-reflowrelayout-restyle/)
+- [Building the DOM faster: speculative parsing, async, defer and preload](https://hacks.mozilla.org/2017/09/building-the-dom-faster-speculative-parsing-async-defer-and-preload/)

front-end/caching.md

@@ -0,0 +1,12 @@
+Caching
+==
+
+## Glossary
+
+- **Cookies**
+
+#### References
+
+- [A Tale of Four Caches](https://calendar.perfplanet.com/2016/a-tale-of-four-caches/)
+- [Web Caching Basics: Terminology, HTTP Headers, and Caching Strategies](https://www.digitalocean.com/community/tutorials/web-caching-basics-terminology-http-headers-and-caching-strategies)
+- [This browser tweak saved 60% of requests to Facebook](https://code.facebook.com/posts/557147474482256/this-browser-tweak-saved-60-of-requests-to-facebook/)

front-end/css.md

@@ -0,0 +1,25 @@
+CSS
+==
+
+CSS (Cascading Style Sheets) are rules to describe how your HTML elements look. Writing good CSS is hard. It usually takes many years of experience and frustration of shooting yourself in the foot before one is able to write maintainable and scalable CSS. CSS, having a global namespace, is fundamentally designed for web documents, and not really for web apps that favor a components architecture. Hence, experienced front end developers have designed methodologies to guide people on how to write organized CSS for complex projects, such as using [SMACSS](https://smacss.com/), [BEM](http://getbem.com/), [SUIT CSS](http://suitcss.github.io/), etc.
+
+However, the encapsulation of styles that these methodologies bring about are artificially enforced by conventions and guidelines. They break the moment developers do not follow them.
+
+As you might have realized by now, the front end ecosystem is saturated with tools, and unsurprisingly, tools have been invented to [partially solve some of the problems](https://speakerdeck.com/vjeux/react-css-in-js) with writing CSS at scale. "At scale" means that many developers are working on the same large project and touching the same stylesheets. There is no community-agreed approach on writing [CSS in JS](https://github.com/MicheleBertoli/css-in-js) at the moment, and we are hoping that one day a winner would emerge, just like Redux did, among all the Flux implementations. For now, I would recommend [CSS Modules](https://github.com/css-modules/css-modules). CSS modules is an improvement over existing CSS that aims to fix the problem of global namespace in CSS; it enables you to write styles that are local by default and encapsulated to your component. This feature is achieved via tooling. With CSS modules, large teams can write modular and reusable CSS without fear of conflict or overriding other parts of the app. However, at the end of the day, CSS modules are still being compiled into normal globally-namespaced CSS that browsers recognize, and it is still important to learn and understand how raw CSS works.
+
+If you are a total beginner to CSS, Codecademy's [HTML & CSS course](https://www.codecademy.com/learn/learn-html-css) will be a good introduction to you. Next, read up on the [Sass preprocessor](http://sass-lang.com/), an extension of the CSS language which adds syntactic improvements and encourages style reusability. Study the CSS methodologies mentioned above, and lastly, CSS modules.
+
+## Glossary
+
+- **Box Model** - The CSS box model describes the rectangular boxes that are generated for elements in the document tree and laid out according to the visual formatting model. Each box has a content area (e.g. text, an image, etc.) and optional surrounding padding, border, and margin areas.
+- **Specificity** - TBD
+- **Positioning** - TBD
+- **Floats** - TBD
+
+## Writing CSS without Side Effects
+
+TODO
+
+###### References
+
+- https://philipwalton.com/articles/side-effects-in-css/

front-end/design.md

@@ -0,0 +1,18 @@
+Design Questions
+==
+
+## Autocomplete Widget
+
+Talk me through a full stack implementation of an autocomplete widget. A user can type text into it, and get back results from a server.
+- How would you design a frontend to support the following features:
+  - Fetch data from a backend API
+  - Render results as a tree (items can have parents/children - it’s not just a flat list)
+  - Support for checkbox, radio button, icon, and regular list items - items come in many forms
+- What does the component's API look like?
+- What does the backend API look like?
+- What perf considerations are there for complete-as-you-type behavior? Are there any edge cases (for example, if the user types fast and the network is slow)?
+- How would you design the network stack and backend in support of fast performance: how do your client/server communicate? How is your data stored on the backend? How do these approaches scale to lots of data and lots of clients?
+
+###### References
+
+- https://performancejs.com/post/hde6d32/The-Best-Frontend-JavaScript-Interview-Questions-%28written-by-a-Frontend-Engineer%29

front-end/dom.md

@@ -0,0 +1,102 @@
+DOM
+==
+
+## Glossary
+
+- **Event Delegation** - Event delegation refers to the process of using event propagation (bubbling) to handle events at a higher level in the DOM than the element on which the event originated. It allows us to attach a single event listener for elements that exist now or in the future.
+
+## Node API
+
+Here's a list of the essential and more common DOM `Node` APIs. It is important to know how to traverse and manipulate the DOM in vanilla JS without jQuery.
+
+**Properties**
+
+- `Node.childNodes` - Returns a live `NodeList` containing all the children of this node. `NodeList` being live means that if the children of the Node change, the `NodeList` object is automatically updated.
+- `Node.firstChild`
+- `Node.lastChild`
+- `Node.nextSibling` - Returns a `Node` representing the next node in the tree, or `null` if there isn't such a node.
+- `Node.nodeName` - `DIV`, `SPAN`, etc. Note that it is in upper case in HTML documents, and has the same value as `Element.tagName`.
+- `Node.parentNode` - Returns a `Node` that is the parent of this node. If there is no such node, like if this node is the top of the tree or if it doesn't participate in a tree, this property returns `null`.
+- `Node.parentElement` - Returns an `Element` that is the parent of this node. If the node has no parent, or if that parent is not an `Element`, this property returns `null`.
+- `Node.previousSibling` - Returns a `Node` representing the previous node in the tree, or `null` if there isn't such a node.
+- `Node.textContent` - Returns / Sets the textual content of an element and all its descendants.
+
+**Methods**
+
+- `Node.appendChild(node)` - Adds the specified `node` argument as the last child to the current node. If the argument referenced an existing node on the DOM tree, the node will be detached from its current position and attached at the new position.
+- `Node.cloneNode(node)` - Clone a `Node`, and optionally, all of its contents. By default, it clones the content of the node.
+- `Node.contains(node)` - Returns a `Boolean` value indicating whether a node is a descendant of a given node or not.
+- `Node.hasChildNodes()` - Returns a `Boolean` indicating if the element has any child nodes, or not.
+- `Node.insertBefore(newNode, referenceNode)` - Inserts the first `Node` before the reference node as a child of the current node. If `referenceNode` is `null`, the `newNode` is inserted at the end of the list of child nodes.
+- `Node.removeChild(node)` - Removes a child node from the current element, which must be a child of the current node.
+- `Node.replaceChild(newChild, oldChild)` - Replaces one child node of the specified node with another node.
+
+## Element API
+
+Here's a list of the essential and more common DOM `Element` APIs. It is important to know how to traverse and manipulate the DOM in vanilla JS without jQuery.
+
+**Properties**
+
+- `Element.attributes` - Returns a `NamedNodeMap` object containing the assigned attributes of the corresponding HTML element.
+- `Element.classList` - Returns a `DOMTokenList` containing the list of class attributes.
+  - `DOMTokenList.add(String [, String])` - Add specified class values. If these classes already exist in attribute of the element, they are ignored.
+  - `DOMTokenList.remove(String [, String])` - Remove specified class values.
+  - `DOMTokenList.toggle(String [, force])` - Toggle specified class value. If second argument is present and evaluates to `true`, add the class value, else remove it.
+  - `DOMTokenList.contains(String)` - Checks if specified class value exists in class attribute of the element.
+- `Element.className` - A `DOMString` representing the class of the element.
+- `Element.id`
+- `Element.innerHTML` - Returns a `DOMString` representing the markup of the element's content or parse the content string and assigns the resulting nodes as children of the element.
+- `Element.tagName` - `DIV`, `SPAN`, etc. Note that it is in upper case in HTML documents, and has the same value as `Node.nodeName`.
+
+**Methods**
+
+- `EventTarget.addEventListener(type, callback, options)` - Registers an event handler to a specific event type on the element. Read up more on the `options` [here](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener).
+- `EventTarget.removeEventListener(type, callback, options)` - Removes an event listener from the element. Read up more on the `options` [here](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/removeEventListener).
+- `Element.closest(selectors)` - Returns the closest ancestor of the current element (or the current element itself) which matches the selectors given in parameter. If there isn't such an ancestor, it returns `null`.
+- `Element.getElementsByClassName(classNames)`- Returns a live `HTMLCollection` that contains all descendants of the current element that possess the list of classes given in the parameter.
+- `Element.getElementsByTagName(tagName)` - Returns a live `HTMLCollection` containing all descendant elements, of a particular tag name, from the current element.
+- `Element.querySelector(selectors)` - Returns the first `Node` which matches the specified selector string relative to the element.
+- `Element.querySelectorAll(selectors)` - Returns a `NodeList` of nodes which match the specified selector string relative to the element.
+- `ChildNode.remove()` - Removes the element from the children list of its parent. TODO: Check whether it's `Element` or `ChildNode`.
+- `Element.setAttribute(attrName, value)` - Sets the value of a named attribute of the current node.
+- `Element.removeAttribute(attrName)` - Removes the named attribute from the current node.
+
+## Document API
+
+- `document.getElementById(id)` - An Element object, or null if an element with the specified ID is not in the document.
+
+## Window/Document Events
+
+- `document.addEventListener('DOMContentLoaded', callback)`
+  - The `DOMContentLoaded` event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. Similar to `jQuery.ready()` but different because `$.ready` will execute immediately if the `DOMContentLoaded` event has already fired.
+  - This corresponds to `document.readyState === 'interactive'`.
+- `window.onload = function() {}`
+  - `window`'s `load` event is only fired after the DOM and all assets have loaded.
+  - This corresponds to `document.readyState === 'complete'`.
+
+## Questions
+
+**What's the difference between `Node.children` and `Node.childNodes`?**
+
+`Node.children` returns a live `HTMLCollection` of the child `elements` of `Node`. `Node.childNodes` returns a `NodeList`, an ordered collection of DOM nodes that are children of the current `Element`. `childNodes` includes all child nodes, including non-element nodes like text and comment nodes. To get a collection of only elements, use `Node.children` instead.
+
+**What's the difference between `NodeList` and `HTMLCollection`?**
+
+A `NodeList` can contain any node type, but an `HTMLCollection` is supposed to only contain `Element` nodes. `HTMLCollection` is always live and is a superset of `NodeList`. `NodeList` need not be live.
+
+**How do you convert an `HTMLCollection` or `NodeList` into an array?**
+
+```
+const nodelist = document.querySelectorAll('div');
+// Array.from
+const divArray = Array.from(nodelist);
+// Array.prototype.slice
+const divArray2 = Array.prototype.slice.call(nodelist); // or .apply
+// ES2015 Spread
+const divArray3 = [...nodeList];
+```
+
+## References
+
+- https://developer.mozilla.org/en-US/docs/Web/API/Node
+- https://developer.mozilla.org/en-US/docs/Web/API/Element

front-end/html.md

@@ -0,0 +1,12 @@
+HTML
+==
+
+## Glossary
+
+- **Doctype**
+
+## Script Loading
+
+- `<script>` - HTML parsing is blocked, the script is fetched and executed immediately, HTML parsing resumes after the script is executed.
+- `<script async>` - The script will be fetched in parallel to HTML parsing and executed as soon as it is available (potentially before HTML parsing completes). Use `async` when the script is independent of any other scripts on the page, for example analytics.
+- `<script defer>` - The script will be fetched in parallel to HTML parsing and executed when the page has finished parsing. If there are multiple of them, each deferred script is executed in the order they were encoun­tered in the document.

front-end/javascript.md

@@ -0,0 +1,99 @@
+JavaScript
+==
+
+## Contents
+
+- [Glossary](#glossary)
+- [Core Language](#core-language)
+- [Design Patterns](#design-patterns)
+- [Strict Mode](#strict-mode)
+
+## Glossary
+
+- **Closure** - TBD
+- **Event Loop** - The event loop is a single-threaded loop that monitors the call stack and checks if there is any work to be done in the message queue. If the call stack is empty and there are callback functions in the message queue, a message is dequeued and pushed onto the call stack to be executed.
+- **Hoisting** - TBD
+- **Promise** - TBD
+- **Prototype** - TBD
+- **This** - The `this` keyword does not refer to the function in which it is used or it’s scope. It refers to the object on which a function is being executed and depends entirely on the call-site of the function.
+
+## Core Language
+
+### Variables
+
+- https://github.com/getify/You-Dont-Know-JS/blob/master/types%20&%20grammar/README.md#you-dont-know-js-types--grammar
+- Types
+- Scopes
+- Coercion
+
+### Functions
+
+- https://github.com/getify/You-Dont-Know-JS/blob/master/scope%20&%20closures/README.md#you-dont-know-js-scope--closures
+- Declaration vs Expression
+- Closures
+- `.call`, `.apply` and `.bind`
+- Currying
+- Arrow functions and lexical this
+
+### Prototypes and Objects
+
+- Reference: https://github.com/getify/You-Dont-Know-JS/blob/master/this%20%26%20object%20prototypes/README.md
+- Prototype chain
+- `this` keyword
+  - https://rainsoft.io/gentle-explanation-of-this-in-javascript/
+  - https://codeburst.io/the-simple-rules-to-this-in-javascript-35d97f31bde3
+- Classes
+  - Methods
+    - Use non-arrow functions for methods that will be called using the `object.method()` syntax because you need the value of `this` to point to the instance itself.
+
+### Async
+
+- Reference: https://github.com/getify/You-Dont-Know-JS/blob/master/async%20&%20performance/README.md#you-dont-know-js-async--performance
+- `setTimeout` and `setInterval`
+- Event Loop
+  -
+- Debounce and Throttle
+  - Throttling enforces a maximum number of times a function can be called over time.
+  - Debouncing enforces that a function not be called again until a certain amount of time has passed without it being called.
+  - https://css-tricks.com/debouncing-throttling-explained-examples/
+- Callbacks
+- Promises
+
+**Reference**
+
+- https://www.vikingcodeschool.com/falling-in-love-with-javascript/the-javascript-event-loop
+
+## Design Patterns
+
+- https://addyosmani.com/resources/essentialjsdesignpatterns/book/
+
+## Strict Mode
+
+1. Strict mode eliminates some JavaScript silent errors by changing them to throw errors.
+1. Strict mode fixes mistakes that make it difficult for JavaScript engines to perform optimizations. Strict mode code can sometimes be made to run faster than identical code that's not strict mode.
+1. Strict mode prohibits some syntax likely to be defined in future versions of ECMAScript.
+
+**Converting Mistakes into Errors**
+
+- Prevent accidental creation of global variables.
+- Makes assignments which would otherwise silently fail throw an exception.
+- Makes attempts to delete undeletable properties throw errors.
+- Requires that all properties named in an object literal be unique. Duplicate property names are a syntax error in strict mode.
+- Requires that function parameter names be unique. In normal code the last duplicated argument hides previous identically-named arguments.
+- Forbids setting properties on primitive values in ES6. Without strict mode, setting properties is simply ignored (no-op), with strict mode, however, a `TypeError` is thrown.
+
+**Simplifying Variable Uses**
+
+- Prohibits `with`.
+- `eval` of strict mode code does not introduce new variables into the surrounding scope.
+- Forbids deleting plain variables. `delete` name in strict mode is a syntax error: `var x; delete x; // !!! syntax error`.
+
+**Paving the way for future ECMAScript versions**
+
+- Future ECMAScript versions will likely introduce new syntax, and strict mode in ECMAScript 5 applies some restrictions to ease the transition. It will be easier to make some changes if the foundations of those changes are prohibited in strict mode.
+- First, in strict mode a short list of identifiers become reserved keywords. These words are `implements`, `interface`, `let`, `package`, `private`, `protected`, `public`, `static`, and `yield`. In strict mode, then, you can't name or use variables or arguments with these names.
+- Second, strict mode prohibits function statements not at the top level of a script or function.
+
+## Useful Links
+
+- https://medium.com/javascript-scene/10-interview-questions-every-javascript-developer-should-know-6fa6bdf5ad95#.l2n8icwl4

front-end/networking.md

@@ -0,0 +1,9 @@
+Networking
+==
+
+## Glossary
+
+- **JSON**
+- **RPC**
+- **HTTP**
+- **HTTP/2**

front-end/performance.md

@@ -0,0 +1,64 @@
+Performance
+==
+
+## Glossary
+
+- **Critical Rendering Path** -
+- `requestAnimationFrame`
+
+## General Strategies
+
+1. Minimize Bytes.
+1. Reduce critical resources.
+1. Reduce CRP length. TODO: Explain what CRP length is.
+
+## Loading
+
+- Minify, Compress, Cache assets.
+- Browsers have a [preloader](https://andydavies.me/blog/2013/10/22/how-the-browser-pre-loader-makes-pages-load-faster/) to load assets ahead of time.
+
+## Rendering
+
+- Remove whitespace and comments from HTML/CSS/JS file via minification.
+- CSS
+  - CSS blocks rendering AND JavaScript execution.
+  - Split up CSS for fewer rendering blocking CSS stylesheets by using media attributes.
+    - Download only the necessary CSS before the browser can start to render.
+    - https://developers.google.com/web/fundamentals/design-and-ui/responsive/#css-media-queries
+  - Use Simpler selectors.
+- JavaScript
+  - JS blocks HTML parsing. If the script is external, it will have to be downloaded first. This incurs latency in network and execution.
+  - Shift `<script>` tags to the bottom.
+  - Async:
+    - Scripts that don't modify the DOM or CSSOM can use the `async` attribute to tell the browser not to block DOM parsing and does not need to wait for the CSSOM to be ready.
+  - Defer JavaScript execution:
+    - There is also a `defer` attribute available. The difference is that with `defer`, the script waits to execute until after the document has been parsed, whereas `async` lets the script run in the background while the document is being parsed.
+  - Use web workers for long running operations to move into a web worker thread.
+  - Use `requestAnimationFrame`
+
+###### References
+
+- https://bitsofco.de/understanding-the-critical-rendering-path/
+
+## Measuring
+
+- [Navigation Timing API](https://developer.mozilla.org/en/docs/Web/API/Navigation_timing_API) is a JavaScript API for accurately measuring performance on the web. The API provides a simple way to get accurate and detailed timing statistics natively for page navigation and load events.
+  - `performance.timing`: An object with the timestamps of the various events on the page. Some uses:
+    - Network latency: `responseEnd` - `fetchStart`.
+    - The time taken for page load once the page is received from the server: `loadEventEnd` - `responseEnd`.
+    - The whole process of navigation and page load: `loadEventEnd` - `navigationStart`.
+
+## Tools
+
+- Yahoo YSlow
+- Google PageSpeed Insights
+- WebPageTest
+- Sitespeed.io
+- Google Lighthouse
+
+## Web Performance Rules
+
+- https://www.html5rocks.com/en/tutorials/webperformance/basics/
+- http://stevesouders.com/hpws/rules.php
+- https://developer.yahoo.com/performance/rules.html
+- https://browserdiet.com/en/

front-end/security.md

@@ -0,0 +1,110 @@
+Security
+==
+
+## Glossary
+
+- **CORS** - Cross-Origin Resource Sharing (CORS).
+- **CSRF** - Cross-Site request forgery (CSRF) is an attack that forces an end user to execute unwanted actions on a web application in which they're currently authenticated.
+- **XSS** - Cross-site scripting (XSS).
+
+## CORS
+
+The same-origin policy protects users by disallowing websites to retrieve information from other websites of different origins. An origin is the triple {protocol, host, port}. Two resources are considered to be of the same origin if and only if all these values are exactly the same.
+
+Cross-Origin Resource Sharing allows relaxing of the same-origin policy. CORS defines a way in which a browser and server can interact to determine whether or not it is safe to allow the cross-origin request.
+
+This standard extends HTTP with a new `Origin` request header and `Access-Control-Allow-Origin` and `Access-Control-Allow-Methods` response headers. It allows servers to use a header to explicitly list origins and HTTP methods that may request a file or to use a wildcard and allow a file to be requested by any site. `XMLHttpRequest`s to a target origin from a different source origin will be blocked if the server did not allow CORS for source origin.
+
+## CSRF
+
+XSS vulnerabilities allow attackers to bypass essentially all CSRF preventions.
+
+#### Protection
+
+- Verifying Same Origin with Standard Headers
+  - There are two steps to this check:
+    1. Determining the origin the request is coming from (source origin).
+    2. Determining the origin the request is going to (target origin).
+  - Examine the `Origin`, `Referer` and `Host` Header values.
+- Synchronizer Tokens
+  - The CSRF token is added as a hidden field for forms or within the URL.
+  - Characteristics of a CSRF Token
+    - Unique per user session
+    - Large random value
+    - Generated by a cryptographically secure random number generator
+  - The server rejects the requested action if the CSRF token fails validation.
+- Double Cookie
+  - When a user visits a site, the site should generate a (cryptographically strong) pseudorandom value and set it as a cookie on the user's machine. The site should require every form submission to include this pseudorandom value as a form value and also as a cookie value. When a POST request is sent to the site, the request should only be considered valid if the form value and the cookie value are the same. When an attacker submits a form on behalf of a user, he can only modify the values of the form. An attacker cannot read any data sent from the server or modify cookie values, per the same-origin policy. This means that while an attacker can send any value he wants with the form, he will be unable to modify or read the value stored in the cookie. Since the cookie value and the form value must be the same, the attacker will be unable to successfully submit a form unless he is able to guess the pseudorandom value.
+  - The advantage of this approach is that it requires no server state; you simply set the cookie value once, then every HTTP POST checks to ensure that one of the submitted <input> values contains the exact same cookie value. Any difference between the two means a possible XSRF attack.
+- Cookie-to-Header Token
+  - On login, the web application sets a cookie containing a random token that remains the same for the whole user session
+    - `Set-Cookie: Csrf-token=i8XNjC4b8KVok4uw5RftR38Wgp2BFwql; expires=Thu, 23-Jul-2015 10:25:33 GMT; Max-Age=31449600; Path=/`
+  - JavaScript operating on the client side reads its value and copies it into a custom HTTP header sent with each transactional request
+    - `X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql`
+  - The server validates presence and integrity of the token.
+  - Security of this technique is based on the assumption that only JavaScript running within the same origin will be able to read the cookie's value.
+  - JavaScript running from a rogue file or email will not be able to read it and copy into the custom header. Even though the `csrf-token` cookie will be automatically sent with the rogue request, the server will be still expecting a valid `X-Csrf-Token` header.
+- Use of Custom Request Headers
+  - An alternate defense which is particularly well suited for AJAX endpoints is the use of a custom request header. This defense relies on the same-origin policy (SOP) restriction that only JavaScript can be used to add a custom header, and only within its origin. By default, browsers don't allow JavaScript to make cross origin requests. Such a header can be `X-Requested-With: XMLHttpRequest`.
+  - If this is the case for your system, you can simply verify the presence of this header and value on all your server side AJAX endpoints in order to protect against CSRF attacks. This approach has the double advantage of usually requiring no UI changes and not introducing any server side state, which is particularly attractive to REST services. You can always add your own custom header and value if that is preferred.
+- Require user interaction
+  - Require a re-authentication, using a one-time token, or requiring users to complete a captcha.
+
+###### References
+
+https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)
+
+## HTTPS
+
+HTTPS is HTTP over SSL/TLS. Servers and clients still speak exactly the same HTTP to each other, but over a secure SSL connection that encrypts and decrypts their requests and responses. The SSL layer has 2 main purposes:
+
+1. Verifying that you are talking directly to the server that you think you are talking to.
+1. Ensuring that only the server can read what you send it and only you can read what it sends back.
+
+#### TLS Handshake
+
+// TODO. Crosscheck and add in more details.
+
+1. The client computer sends a `ClientHello` message to the server with its Transport Layer Security (TLS) version, list of cipher algorithms and compression methods available.
+1. The server replies with a `ServerHello` message to the client with the TLS version, selected cipher, selected compression methods and the server's public certificate signed by a CA (Certificate Authority). The certificate contains a public key that will be used by the client to encrypt the rest of the handshake until a symmetric key can be agreed upon.
+1. The client verifies the server digital certificate against its list of trusted CAs. If trust can be established based on the CA, the client generates a string of pseudo-random bytes and encrypts this with the server's public key. These random bytes can be used to determine the symmetric key.
+1. The server decrypts the random bytes using its private key and uses these bytes to generate its own copy of the symmetric master key.
+1. The client sends a `Finished` message to the server, encrypting a hash of the transmission up to this point with the symmetric key.
+1. The server generates its own hash, and then decrypts the client-sent hash to verify that it matches. If it does, it sends its own `Finished` message to the client, also encrypted with the symmetric key.
+1. From now on the TLS session transmits the application (HTTP) data encrypted with the agreed symmetric key.
+
+#### Downsides of HTTPS
+
+- TLS handshake computational and latency overhead.
+- Encryption and decryption requires more computation power and bandwidth.
+
+###### References
+
+- https://blog.hartleybrody.com/https-certificates/
+- https://github.com/alex/what-happens-when#tls-handshake
+- http://robertheaton.com/2014/03/27/how-does-https-actually-work/
+
+## XSS
+
+XSS vulnerabilities allow attackers to bypass essentially all CSRF preventions.
+
+```js
+const name = "<img src='x' onerror='alert(1)'>";
+el.innerHTML = name;
+```
+
+http://shebang.brandonmintern.com/foolproof-html-escaping-in-javascript/
+
+## Session hijacking
+
+- https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies
+- https://www.nczonline.net/blog/2009/05/12/cookies-and-security/
+
+
+## Framebusting
+
+https://seclab.stanford.edu/websec/framebusting/framebust.pdf
+
+## API
+
+https://github.com/shieldfy/API-Security-Checklist

front-end/widgets.md

@@ -0,0 +1,28 @@
+Widgets
+==
+
+Here are a list of commonly seen widgets/components and the issues/pitfalls with them.
+
+## Autocomplete
+
+TODO
+
+## Carousel
+
+TODO
+
+## Dropdown
+
+TODO
+
+## Modal
+
+- When dismissing, if a click handler is attached and you click on the modal content, the backdrop might receive the click be triggered as a result.
+
+###### References
+
+- https://css-tricks.com/dangers-stopping-event-propagation/
+
+## Tooltip
+
+TODO

non-technical/behavioral.md

@@ -0,0 +1,156 @@
+Behavioral
+==
+
+Learn the [STAR](https://en.wikipedia.org/wiki/Situation,_task,_action,_result) format. From Wikipedia:
+
+- **Situation** - The interviewer wants you to present a recent challenge and situation in which you found yourself.
+- **Task** - What were you required to achieve? The interviewer will be looking to see what you were trying to achieve from the situation. Some performance development methods[1] use “Target” rather than “Task”. Job interview candidates who describe a “Target” they set themselves instead of an externally imposed “Task” emphasize their own intrinsic motivation to perform and to develop their performance.
+- **Action** - What did you do? The interviewer will be looking for information on what you did, why you did it and what the alternatives were.
+- **Results** - What was the outcome of your actions? What did you achieve through your actions and did you meet your objectives? What did you learn from this experience and have you used this learning since?
+
+## General
+
+- Why do you want to work for X company?
+- Why do you want to leave your current/last company?
+- Tell me about a time where you had a conflict with a co-worker.
+- Tell me about a time in which you had a conflict and needed to influence somebody else.
+- What project are you currently working on?
+- What is the most challenging aspect of your current project?
+- What was the most difficult bug that you fixed in the past 6 months?
+- How do you tackle challenges? Name a difficult challenge you faced while working on a project, how you overcame it, and what you learned.
+- What are you excited about?
+– Imagine it is your first day here at the company. What do you want to work on? What features would you improve on?
+– What are the most interesting projects you have worked on and how might they be relevant to the this company's environment?
+- Tell me about a time you had a disagreement with your manager.
+- Talk about a project you are most passionate about, or one where you did your best work.
+- What does your best day of work look like?
+
+## Airbnb
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Airbnb-Interview-Questions-E391850.htm)
+
+While loving to travel or appreciating Airbnb's growth may be good answers, try to demonstrate the deep connection you have with the product.
+
+- What does it mean to be you belong anywhere?
+- What large problems in the world would you solve today?
+- Why do you like Airbnb?
+- If you had an unlimited budget and you could buy one gift for one person, what would you buy and who would you buy it for?
+- Share one of your trips with us.
+- What is the most challenging project in or out of school that you have worked on in the last 6 months.
+- What is the thing that you don't want from your last internship/job?
+- Give me an example of when you've been a good host.
+- One thing you would like to remove from the Airbnb experience.
+- What is something new that you can teach your interviewer in a few minutes?
+- Tell me about why you want to work here.
+- What is the best gift you have ever given or received?
+- Tell me about a time you were uncomfortable and how you dealt with it.
+- Explain a project that you worked on recently.
+- What do you think of Airbnb?
+- Tell me something about yourself and why you'd be a good fit for the position.
+- Name a situation where you were impressed by a company's customer service.
+- How did you work with senior management on large projects as well as multiple internal teams?
+- Tell me about a time you had to give someone terrible news.
+- If you were a gerbil, which gerbil would you be?
+- What excites you about the company?
+- How does Airbnb impact our guests and hosts?
+- What part of our mission resonates the most with you?
+
+## Amazon
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Amazon-Interview-Questions-E6036.htm)
+
+- How do you deal with a failed deadline?
+- Why do you want to work for Amazon?
+- Talked about a situation where you had a conflict with a teammate.
+- In my professional experience have you worked on something without getting approval from your manager?
+- Tell me a situation where you would have done something differently from what you actually did.
+
+## Dropbox
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Dropbox-Interview-Questions-E415350.htm)
+
+- Talk about your favorite project.
+- If you were hired here what would you do?
+- State an experience about how you solved a technical problem. Be specific about the diagnosis and process.
+
+## Hired
+
+Source: [Glassdoor](https://hired.com/blog/candidates/10-top-interview-questions-how-to-answer/)
+
+- Tell me about yourself.
+- What is your biggest strength and area of growth?
+- Why are you interested in this opportunity?
+- What are your salary expectations?
+- Why are you looking to leave your current company?
+- What is your biggest strength and area of growth?
+- Tell me about a time your work responsibilities got a little overwhelming. What did you do?
+- Give me an example of a time where you had a difference of opinion with a team member. How did you handle that?
+- Tell me about a challenge you faced recently in your role. How did you tackle it? What was the outcome?
+- Where do you want to be in five years?
+- Tell me about a time you needed information from someone who wasn’t responsive. What did you do?
+
+## Lyft
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Lyft-Interview-Questions-E700614.htm)
+
+- Tell me about your most interesting/challenging project to date.
+- Why Lyft? What are you looking for in the next role?
+
+## Palantir
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Palantir-Technologies-Interview-Questions-E236375.htm)
+
+- How do you deal with difficult coworkers? Think about specific instances where you resolved conflicts.
+- How did you win over the difficult employees?
+- Tell me about an analytical problem that you have worked on in the past.
+- What are your three strengths and three weaknesses?
+- If you were in charge of picking projects for Palantir, what problem would you try to solve?
+- **What is something 90% of people disagree with you about?**
+- What are some of the best and worse things about your current company?
+- **What is broken around you?**
+- What would your manager say about you?
+- Describe Palantir to your grandmother.
+- Teach me something you've learned?
+- Tell me a time when you predicted something?
+- If your supervisors were to rate you on a scale of 1-10 what would they rate you?
+- What was the most fun thing you did recently?
+- Tell me the story of how you became who you are today and what made you apply to Palantir.
+
+## Slack
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Slack-Interview-Questions-E950758.htm)
+
+- Tell me something about your internship.
+- Why do you want to join Slack?
+- Tell me about your past projects.
+- Explain me your toughest project and the working architecture.
+- Apart from technical knowledge what did you learn during your internship?
+- If someone has a different viewpoint to do a project like different programming language, how would handle this situation?
+- What are your most interesting subjects and why?
+- Did you find any bug in Slack?
+- What is your favorite feature and why?
+
+## Stack Overflow
+
+Source: [Glassdoor](https://hired.com/blog/candidates/10-top-interview-questions-how-to-answer/)
+
+- What have you built?
+- What is the hardest technical problem you have run into?
+- How did you solve it?
+- Where do you see yourself in 5 years?
+- Why do you want to work here?
+- How do you handle disagreements with coworkers?
+
+## Stripe
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Stripe-Interview-Questions-E671932.htm)
+
+- How do you stay up to date with latest technologies?
+- Explain a project that you worked on recently that was difficult.
+- Where do you see yourself in five years?
+
+## Twitter
+
+Source: [Glassdoor](https://www.glassdoor.com/Interview/Twitter-Interview-Questions-E100569.htm)
+
+- What would your previous boss say your biggest strength was?

non-technical/cover-letter.md

@@ -0,0 +1,8 @@
+Cover Letter
+==
+
+- A short introduction describing who you are and what you’re looking for.
+- What projects have you enjoyed working on?
+- Which have you disliked? What motivates you?
+- Links to online profiles you use (GitHub, Twitter, etc).
+- A description of your work history (whether as a resume, LinkedIn profile, or prose).

non-technical/format.md

@@ -0,0 +1,130 @@
+Interview Formats
+==
+
+The following interview formats are based on my experience interviewing with Bay Area companies. Formats would differ slightly depending on the roles you are applying to. Many companies like to use [CoderPad](https://coderpad.io/) for collaborative code editing. CoderPad supports running of the program, so you might be asked to fix your code such that it can be run. For front end interviews, many companies like to use [CodePen](https://codepen.io/), and it will be worth your time to familiarize yourself with the user interfaces of such web-based coding environments.
+
+For on-site interviews at smaller (non-public) companies, most will allow (and prefer) that you use your own laptop. Hence it is important that you prepare your development environment in advance.
+
+## Companies
+
+- [Airbnb](#airbnb)
+- [Asana](#asana)
+- [Dropbox](#dropbox)
+- [Facebook](#facebook)
+- [Google](#google)
+- [Lyft](#lyft)
+- [Palantir](#palantir)
+- [WhatsApp](#whatsapp)
+
+### Airbnb
+
+- Recruiter phone screen.
+- Technical phone interview:
+  - 1 or 2 x Algorithm/front end on CoderPad/CodePen.
+- On-site (General):
+  - 2 x Algorithm coding on CoderPad.
+  - 1 x System design/architecture.
+  - 1 x Past experience/project.
+  - 2 x Cross functional.
+- On-site (Front End):
+  - 2 x Front end coding on CodePen. Use any framework/library.
+  - 1 x General coding on your own laptop.
+  - 1 x Past experience/project.
+  - 2 x Cross functional.
+- Tips:
+  - All sessions involve coding on your own laptop. Prepare your development environment in advance.
+  - You are allowed to look up APIs if you need to.
+  - They seem to place high emphasis on compilable, runnable code in all their coding rounds.
+  - Cross functional interviews will involve getting Airbnb employees from any discipline to speak with you. These interviews are mostly non-technical but are extremely important to Airbnb because they place a high emphasis on cultural fit. Do look up the Airbnb section of the behavioural questions to know what sort of questions to expect.
+
+### Asana
+
+- Recruiter phone screen.
+- Technical phone interview.
+- On-site (Product Engineer):
+  - 3 x Algorithm and system design on whiteboard within the same session.
+  - 1 x Algorithm on laptop and system design. This session involves writing code on your own laptop to solve 3 well-defined algorithm problems in around 45 minutes after which an engineer will come in and review the code with you. You are not supposed to run the code while working on the problem.
+- Tips:
+  - No front end questions were asked.
+  - Asana places high emphasis on System Design and makes heavy use of the whiteboard. You do not necessarily have to write code for the algorithm question of the first three interviews.
+  - All 4 sessions involve algorithms and system design. One of the sessions will be conducted by an Engineering Manager.
+  - The last session will involve coding on your own laptop. Prepare your development environment in advance.
+  - Regardless of Product Engineer or Engineering Generalist position, their interview format and questions are similar.
+
+### Dropbox
+
+- Recruiter phone screen.
+- Technical phone interviews:
+  - 2 x Algorithm/front end on CoderPad/CodePen.
+- On-site (Front End):
+  - 2 x Front end on CodePen. Only Vanilla JS or jQuery allowed.
+  - 1 x General coding on CoderPad.
+  - 1 x All around. Meet with an Engineering Manager and discussing past experiences and working style.
+- Tips:
+  - You can code on your own laptop and look up APIs.
+  - Dropbox recruiters are very nice and will give you helpful information on what kind of questions to expect for the upcoming sessions.
+  - One of the front end sessions involve coding up a pixel-perfect version of a real page on www.dropbox.com. You'll be given a spec of the desired page and you'll be asked to create a working version during the interview.
+
+### Facebook
+
+- Recruiter phone screen.
+- Technical phone interviews:
+  - 1 or 2 x Algorithm/front end on Skype/CoderPad.
+- On-site (Front End):
+  - 2 x Technical coding interview on whiteboard (Ninja).
+  - 1 x Behavioural (Jedi). Meet with an Engineering Manager and discussing past experiences and working style.
+  - 1 x Design/architecture on whiteboard (Pirate).
+- Tips:
+  - You are only allowed to use the whiteboard. No laptops involved.
+  - For the Jedi round, you may be asked a technical question at the end of it. Front end candidates will be given a small HTML/CSS problem nearing the end of the session.
+  - For the Ninja rounds, you may be asked one to two questions depending on how fast you progress through the question.
+
+### Google
+
+- Recruiter phone screen.
+- Technical phone interview:
+  - 1 or 2 x algorithm on Google Doc.
+- On-site (Front End):
+  - 3 x Front end on whiteboard. Have to use Vanilla JS (or at the most, jQuery).
+  - 2 x Algorithm on whiteboard.
+- Tips:
+  - You are only allowed to use the whiteboard. No laptops involved.
+  - In rare cases, candidates may even be allowed to skip the phone interview round and advanced to on-site directly.
+
+### Lyft
+
+- Recruiter phone screen.
+- Technical phone interview:
+  - 1 x Algorithm/Front end over JSFiddle.
+- On-site (Front End):
+  - 4 x Front end on Coderpad/your own laptop. Use any language/framework.
+  - 1 x Behavioural. Meet with an Engineering Manager and go through candidate's resume.
+- Tips:
+  - Can use whiteboard and/or laptop.
+  - For front end coding, I opted to use React and had to set up the projects on the spot using `create-react-app`.
+
+### Palantir
+
+- Recruiter phone screen.
+- Technical phone interview:
+  - 1 x Algorithm over HackerRank CodePair and Skype.
+- On-site (General):
+  - 2 x Algorithm on whiteboard.
+  - 1 x Decomposition (system design) on whiteboard.
+- On-site (Front End):
+  - 1 x Front end on your own laptop. This session lasts about 1.5 hours. Use any library/framework.
+  - 1 x Decomposition (system design) on whiteboard.
+- Tips:
+  - I opted to use React and had to set up projects on the spot using `create-react-app`.
+  - You may be asked to meet with Engineering Managers after the technical sessions and it's not necessarily a good/bad thing.
+
+### WhatsApp
+
+- Recruiter phone screen.
+- Technical phone interview:
+  - 2 x Algorithm over CoderPad.
+- On-site (Web Client Developer):
+  - 4 x Algorithm on whiteboard.
+- Tips:
+  - No front end questions were asked.
+  - 1 of the interviewers is an Engineering Manager.

non-technical/negotiation.md

@@ -0,0 +1,86 @@
+Negotiation
+==
+
+### Ten Rules of Negotiation
+
+Key points extracted from "Ten Rules for Negotiating a Job Offer" [Part 1](http://haseebq.com/my-ten-rules-for-negotiating-a-job-offer/) and [Part 2](https://haseebq.com/how-not-to-bomb-your-offer-negotiation/) by Haseeb Qureshi.
+
+#### Get everything in writing
+
+Note down EVERYTHING on your phone call with the recruiters as they may be helpful later on. Even if there are things that are not directly monetary, if they relate to the job, write them down. If they tell you "we're working on porting the front-end to Angular," write that down. If they say they have 20 employees, write that down. You want as much information as you can. You'll forget a lot of this stuff, and it's going to be important in informing your final decision.
+
+#### Always keep the door open
+
+Never give up your negotiating power until you're absolutely ready to make an informed, deliberate final decision. This means your job is to traverse as many of these decision points as possible without giving up the power to continue negotiating. Very frequently, your interlocutor will try to trick you into making a decision, or tie you to a decision you didn't commit to. You must keep verbally jiu-jitsu-ing out of these antics until you're actually ready to make your final decision.
+
+#### Information is power
+
+To protect your power in the negotiation, you must protect information as much as possible. A corollary of this rule is that you should not reveal to companies what you're currently making. So given this offer, don't ask for more money or equity or anything of the sort. Don't comment on any specific details of the offer except to clarify them. Companies will ask about your current compensation at different stages in the process—some before they ever interview you, some after they decide to make you an offer. But be mindful of this, and protect information.
+
+> "Yeah, [COMPANY_NAME] sounds great! I really thought this was a good fit, and I'm glad that you guys agree. Right now I'm talking with a few other companies so I can't speak to the specific details of the offer until I'm done with the process and get closer to making a decision. But I'm sure we'll be able to find a package that we're both happy with, because I really would love to be a part of the team."
+
+#### Always be positive
+
+Even if the offer is bad, it's extremely important to remain positive and excited about the company. This is because your excitement is one of your most valuable assets in a negotiation.
+
+Despite whatever is happening in the negotiation, give the company the impression that 1) you still like the company, and that 2) you're still excited to work there, even if the numbers or the money or the timing is not working out. Generally the most convincing thing to signal this is to reiterate you love the mission, the team, or the problem they're working on, and really want to see things work out.
+
+#### Don't be the decision maker
+
+Even if you don't particularly care what your friends/family/husband/mother thinks, by mentioning them, you're no longer the only person the recruiter needs to win over. There's no point in them trying to bully and intimidate you; the "true decision-maker" is beyond their reach. This is a classic technique in customer support and remediation. It's never the person on the phone's fault, they're just some poor schmuck doing their job. It's not their decision to make. This helps to defuse tension and give them more control of the situation.
+
+> I'll look over some of these details and discuss it with my [FAMILY/CLOSE_FRIENDS/SIGNIFICANT_OTHER]. I'll reach out to you if I have any questions. Thanks so much for sharing the good news with me, and I'll be in touch!
+
+It's much harder to pressure someone if they're not the final decision-maker. So take advantage of that.
+
+#### Have alternatives
+
+If you're already in the pipeline with other companies (which you should be if you're doing it right), you should proactively reach out and let them know that you've just received an offer. Try to build a sense of urgency. Regardless of whether you know the expiration date, all offers expire at some point, so take advantage of that.
+
+> Hello [PERSON],
+>
+> I just wanted to update you on my own process. I've just received an offer from [COMPANY] which is quite strong. That said, I'm really excited about [YOUR AMAZING COMPANY] and really want to see if we can make it work. Since my timeline is now compressed, is there anything you can do to expedite the process?
+
+Should you specifically mention the company that gave you an offer? Depends. If it's a well-known company or a competitor, then definitely mention it. If it's a no-name or unsexy company, you should just say you received an offer. If it's expiring soon, you should mention that as well.
+
+Either way, send out a letter like this to every single company you're talking to. No matter how hopeless or pointless you think your application is, you want to send this signal to everyone who is considering you in the market.
+
+Companies care that you've received other offers. They care because each company knows that their own process is noisy, and the processes of most other companies are also noisy. But a candidate having multiple offers means that they have multiple weak signals in their favor. Combined, these converge into a much stronger signal than any single interview. It's like knowing that a student has a strong SAT score, and GPA, and won various scholarships. Sure, it's still possible that they're a dunce, but it's much harder for that to be true.
+
+This is not to say that companies respond proportionally to these signals, or that they don't overvalue credentials and brands. They do. But caring about whether you have other offers and valuing you accordingly is completely rational.
+
+Tell other companies that you've received offers. Give them more signals so that they know you're a valued and compelling candidate. And understand why this changes their mind about whether to interview you.
+
+Your goal should be to have as many offers overlapping at the same time as possible. This will maximize your window for negotiating.
+
+Have a strong BATNA (Best Alternative To a Negotiated Agreement) and communicate it.
+
+> I 've received another offer from [OTHER CORP] that's very compelling on salary, but I really love the mission of [YOUR COMPANY] and think that it would overall be a better fit for me.
+
+> I'm also considering going back to grad school and getting a Master's degree in Postmodern Haberdashery. I'm excited about [YOUR COMPANY] though and would love to join the team, but the package has to make sense if I'm going to forego a life of ironic hatmaking.
+
+#### Proclaim reasons for everything
+
+It's kind of a brain-hack, both for yourself and for your negotiating partner. Just stating a reason (any reason) makes your request feel human and important. It's not you being greedy, it's you trying to fulfill your goals.
+
+The more unobjectionable and sympathetic your reason, the better. If it's medical expenses, or paying off student loans, or taking care of family, you'll bring tears to their eyes.
+
+Just go with it, state a reason for everything, and you'll find recruiters more willing to become your advocate.
+
+#### Be motivated by more than just money
+
+You should be motivated by money too of course, but it should be one among many dimensions you're optimizing for. How much training you get, what your first project will be, which team you join, or even who your mentor will be—these are all things you can and should negotiate.
+
+Of course, to negotiate well you need to understand the other side's preferences. You want to make the deal better for both of you.
+
+#### Understand what they value
+
+Remember that you can always get salary raises as you continue to work at the company, but there's only one point at which you can get a signing bonus.
+
+The easiest thing for a company to give though is stock (if the company offers stock). Companies like giving stock because it invests you in the company and aligns interests. It also shifts some of the risk from the company over to you and burns less cash.
+
+#### Be winnable
+
+This is more than just giving the company the impression that you like them (which you continually should). But more so that you must give any company you're talking to a clear path on how to win you. Don't bullshit them or play stupid games. Be clear and unequivocal with your preferences and timeline.
+
+Don't waste their time or play games for your own purposes. Even if the company isn't your dream company, you must be able to imagine at least some package they could offer you that would make you sign. If not, politely turn them down.

non-technical/psychological.md

@@ -0,0 +1,27 @@
+Psychological Tricks
+==
+
+Here are some psychological tricks that will help you ace a job interview.
+
+- Tailor your answers to the interviewer’s age.
+  - Generation Y interviewers (between 20 and 30): Bring along visual samples of your work and highlight your ability to multitask.
+  - Generation X interviewers (between 30 and 50): Emphasize your creativity and mention how work/life balance contributes to your success.
+  - Baby Boomer interviewers (between 50 and 70): Show that you work hard and demonstrate respect for what they've achieved.
+- Hold your palms open or steeple your hands.
+- Find something in common with your interviewer.
+- Mirror the interviewer’s body language.
+- Compliment the interviewer and the organization without self-promoting.
+  - Specifically, the students who ingratiated themselves praised the organization and indicated their enthusiasm for working there, and complimented the interviewer. They didn't play up the value of positive events they took credit for or take credit for positive events even if they weren’t solely responsible.
+- Show confidence and deference simultaneously.
+  - In a job interview, that means showing deference to your interviewer, while also demonstrating self-confidence. One way to do that is to say something like, "I love your work on [whatever area]. It reminds me of my work on [whatever area]."
+- Emphasize how you took control of events in your previous jobs.
+  - To impress your interviewer, you should talk about past work experiences where you took initiative.
+- Be candid about your weaknesses.
+  - It's wiser to say something genuine like, "I'm not always the best at staying organized," which sounds more honest, and could make your interviewer more inclined to recommend you for the position.
+- Speak expressively.
+- Showcase your potential.
+  - You might be tempted to tell your interviewer all about your past accomplishments — but research suggests you should focus more on what you could do in the future, if the organization hires you.
+
+###### References
+
+- [Business Insider](http://www.businessinsider.com/psychological-tricks-to-ace-job-interview-2015-11)

non-technical/questions-to-ask.md

@@ -0,0 +1,74 @@
+Questions to Ask
+==
+
+Here are some good questions to ask at the end of the interview, extracted from various sources. The ones in **bold** are the ones that I like 😄.
+
+## General
+
+- How do your clients and customers define success?
+- What would you change around here if you could?
+- What are some weaknesses of the organization?
+- What does a typical day look like for you?
+- **What are you most proud about in your career so far?**
+- What do you think the company can improve at?
+- What is the most important/valuable thing you have learnt from working here?
+- How would you see yourself growing at this company in the next few years?
+- What do you wish you were told about the company before you joined?
+
+## Culture
+
+- What was your best moment so far at the company?
+- What do you like most about working here?
+- **What is the most frustrating part about working here?**
+- **How will the work I will be doing contribute to the organization's mission?**
+- How are you improving diversity?
+- When you were last interviewing, what were some of your other options, and what made you choose this company?
+- What is something you wish were different about your job?
+- How often have you moved teams? What made you join the team you’re on right now? If you wanted to move teams, what would need to happen?
+- (If the company is a startup) When's the last time you interacted with a founder? What was it regarding? Generally how involved are the founders in the day-to-day?
+- Does the company culture encourage entrepreneurship? Could you give me any specific examples?
+- What was something you wish someone would have told you before you joined?
+- What is unique about working at this company that you have not experienced elsewhere?
+- What does the company do to nurture and train its employees?
+
+## Technical
+
+- How would you describe your engineering culture?
+- **What are the engineering challenges that the company/team is facing?**
+- How has your role changed since joining the company?
+- What is your stack? What is the rationale for/story behind this specific stack?
+- Do you tend to roll your own solutions more often or rely on third party tools? What’s the rationale in a specific case?
+- **What has been the worst technical blunder that has happened in the recent past? How did you guys deal with it? What changes were implemented afterwards to make sure it didn't happen again?**
+- **What is the most costly technical decision made early on that the company is living with now?**
+- How does the engineering team balance resources between feature requests and engineering maintenance?
+- **What is the most fulfilling/exciting/technically complex project that you've worked on here so far?**
+- **How do you train/ramp up engineers who are new to the team?**
+
+## Behavioral
+
+- Was there a time where you messed up and how was it handled?
+- How would you see yourself growing at this company in the next few years?
+
+## HR
+
+- What concerns/reservations do you have about me for this position?
+- What can I help to clarify that would make hiring me an easy decision?
+- **How do you see this position evolving in the next three years?**
+- **Who is your ideal candidate and how can I make myself more like them?**
+- How does the management team deal with mistakes?
+- If you could hire anyone to join your team, who would that be and why?
+
+## Attrition
+
+- How long does the average engineer stay at the company?
+- Why have the last few people left?
+- Have you ever thought about leaving? If you were to leave, where would you go?
+
+#### References
+
+- [Business Insider](http://www.businessinsider.sg/impressive-job-interview-questions-2015-3/)
+- [Lifehacker](http://lifehacker.com/ask-this-question-to-end-your-job-interview-on-a-good-n-1787624433)
+- [Fastcompany](https://www.fastcompany.com/40406730/7-questions-recruiters-at-amazon-spotify-and-more-want-you-to-ask)
+- [Questions I'm asking in interviews](http://jvns.ca/blog/2013/12/30/questions-im-asking-in-interviews/)
+- [How to interview your interviewers](http://blog.alinelerner.com/how-to-interview-your-interviewers/)
+- [How to Break Into the Tech Industry—a Guide to Job Hunting and Tech Interviews](https://haseebq.com/how-to-break-into-tech-job-hunting-and-interviews/)

non-technical/self-introduction.md

@@ -0,0 +1,10 @@
+Self Introduction
+==
+
+You can rephrase the question like this:
+
+"Tell me about your journey into tech. How did you get interested in coding, and why was web development a good fit for you? How is that applicable to our _____ role or company goals?"
+
+###### References
+
+- [8 Secrets to Software Engineer Self Introduction](http://blog.gainlo.co/index.php/2016/10/14/8-secretes-software-engineer-self-introduction)

utilities/binarySearch.js

@@ -0,0 +1,26 @@
+function binarySearch(arr, target) {
+    let left = 0;
+    let right = arr.length - 1;
+    while (left < right) {
+        let mid = left + Math.floor((right - left) / 2);
+        if (arr[mid] === target) {
+            return mid;
+        }
+        if (arr[mid] < target) {
+            left = mid + 1;
+        } else {
+            right = mid - 1;
+        }
+    }
+    return left;
+}
+
+console.log(binarySearch([1, 2, 3, 10], 1) === 0)
+console.log(binarySearch([1, 2, 3, 10], 2) === 1)
+console.log(binarySearch([1, 2, 3, 10], 3) === 2)
+console.log(binarySearch([1, 2, 3, 10], 10) === 3)
+console.log(binarySearch([1, 2, 3, 10], 9) === 3)
+console.log(binarySearch([1, 2, 3, 10], 4) === 3)
+console.log(binarySearch([1, 2, 3, 10], 0) === 0)
+console.log(binarySearch([1, 2, 3, 10], 11) === 3)
+console.log(binarySearch([5, 7, 8, 10], 3) === 0)

utilities/binary_search.py

@@ -0,0 +1,22 @@
+def binary_search(arr, target):
+    left = 0;
+    right = len(arr) - 1
+    while left < right:
+        mid = left + (right - left) / 2;
+        if arr[mid] == target:
+            return mid
+        elif arr[mid] < target:
+            left = mid + 1
+        else:
+            right = mid - 1
+    return left
+
+print(binary_search([1, 2, 3, 10], 1) == 0)
+print(binary_search([1, 2, 3, 10], 2) == 1)
+print(binary_search([1, 2, 3, 10], 3) == 2)
+print(binary_search([1, 2, 3, 10], 10) == 3)
+print(binary_search([1, 2, 3, 10], 9) == 3)
+print(binary_search([1, 2, 3, 10], 4) == 3)
+print(binary_search([1, 2, 3, 10], 0) == 0)
+print(binary_search([1, 2, 3, 10], 11) == 3)
+print(binary_search([5, 7, 8, 10], 3) == 0)

utilities/char_prime_map.py

@@ -0,0 +1,19 @@
+# For mapping a lowercase character to a prime number.
+# Useful for checking whether two strings are anagram or permutations of each other.
+primes = {
+    'a': 2, 'b': 3, 'c': 5, 'd': 7, 'e': 11, 'f': 13,
+    'g': 17, 'h': 19, 'i': 23, 'j': 29, 'k': 31, 'l': 37,
+    'm': 41, 'n': 43, 'o': 47, 'p': 53, 'q': 59, 'r': 61,
+    's': 67, 't': 71, 'u': 73, 'v': 79, 'w': 83, 'x': 89,
+    'y': 97, 'z': 101, ' ': 103,
+}
+
+import functools
+
+def mul(seq):
+    return functools.reduce(lambda a, b: a * b, seq, 1)
+
+def prime_value_of_string(string):
+    return mul([primes[c] for c in string])
+
+print(prime_value_of_string('abcde'))

utilities/graphTopoSort.js

@@ -0,0 +1,35 @@
+function graphTopoSort(numberNodes, edges) {
+    const nodes = new Map();
+    const order = [];
+    const queue = [];
+    for (let i = 0; i < numberNodes; i++) {
+        nodes.set(i, { in: 0, out: new Set() });
+    }
+
+    edges.forEach(edge => {
+        const [node_id, pre_id] = edge;
+        nodes.get(node_id).in += 1;
+        nodes.get(pre_id).out.add(node_id);
+    });
+
+    for (let [node_id, value] of nodes.entries()) {
+        if (value.in === 0) {
+            queue.push(node_id);
+        }
+    }
+
+    while (queue.length) {
+        const node_id = queue.shift();
+        for (let outgoing_id of nodes.get(node_id).out) {
+            nodes.get(outgoing_id).in -= 1;
+            if (nodes.get(outgoing_id).in === 0) {
+                queue.push(outgoing_id);
+            }
+        }
+        order.push(node_id);
+    }
+
+    return order.length == numberNodes ? order : [];
+}
+
+console.log(graphTopoSort(3, [[0, 1], [0, 2]]))

utilities/graph_dfs.py

@@ -0,0 +1,24 @@
+def graph_dfs(matrix):
+    rows, cols = len(matrix), len(matrix[0])
+    visited = set()
+    directions = ((0, 1), (0, -1), (1, 0), (-1, 0))
+    def dfs(i, j):
+        if (i, j) in visited:
+            return
+        visited.add((i, j))
+        # Traverse neighbors.
+        for direction in directions:
+            next_i, next_j = i + direction[0], j + direction[1]
+            if 0 <= next_i < rows and 0 <= next_j < cols: # Check boundary.
+                # Add any other checking here ^
+                dfs(next_i, next_j)
+
+    for i in range(rows):
+        for j in range(cols):
+            dfs(i, j)
+
+graph_dfs([
+    [1, 2, 3, 4],
+    [5, 6, 7, 8],
+    [9, 10, 11, 12],
+])

utilities/graph_dijkstra.py

@@ -0,0 +1 @@
+# TODO

utilities/graph_topo_sort.py

@@ -0,0 +1,21 @@
+def graph_topo_sort(num_nodes, edges):
+    from collections import deque
+    nodes, order, queue = {}, [], deque()
+    for node_id in range(num_nodes):
+        nodes[node_id] = { 'in': 0, 'out': set() }
+    for node_id, pre_id in edges:
+        nodes[node_id]['in'] += 1
+        nodes[pre_id]['out'].add(node_id)
+    for node_id in nodes.keys():
+        if nodes[node_id]['in'] == 0:
+            queue.append(node_id)
+    while len(queue):
+        node_id = queue.pop()
+        for outgoing_id in nodes[node_id]['out']:
+            nodes[outgoing_id]['in'] -= 1
+            if nodes[outgoing_id]['in'] == 0:
+                queue.append(outgoing_id)
+        order.append(node_id)
+    return order if len(order) == num_nodes else []
+
+print(graph_topo_sort(3, [[0, 1], [0, 2]]))

utilities/isSubsequence.js

@@ -0,0 +1,19 @@
+function isSubsequence(s, t) {
+    if (s.length > t.length) {
+        return false;
+    }
+    let matchedLength = 0;
+    for (let i = 0; i < t.length; i++) {
+        if (matchedLength < s.length && s[matchedLength] === t[i]) {
+            matchedLength += 1;
+        }
+    }
+    return matchedLength === s.length;
+}
+
+console.log(isSubsequence('abc', 'abcde') === true);
+console.log(isSubsequence('abd', 'abcde') === true);
+console.log(isSubsequence('abf', 'abcde') === false);
+console.log(isSubsequence('abef', 'abcde') === false);
+console.log(isSubsequence('abcdef', 'abcde') === false);
+console.log(isSubsequence('a', 'abcde') === true);

utilities/is_subsequence.py

@@ -0,0 +1,13 @@
+def is_subsequence(s, t):
+    """
+    :type s: str
+    :type t: str
+    :rtype: bool
+    """
+    if len(s) > len(t):
+        return False
+    matched_s = 0
+    for char in t:
+        if matched_s < len(s) and s[matched_s] == char:
+            matched_s += 1
+    return matched_s == len(s)

utilities/string_rabin_karp.py

@@ -0,0 +1 @@
+# TODO

utilities/treeEqual.js

@@ -0,0 +1,11 @@
+function treeEqual(node1, node2) {
+    if (!node1 && !node2) {
+        return true;
+    }
+    if (!node1 || !node2) {
+        return false;
+    }
+    return node1.val == node2.val &&
+        treeEqual(node1.left, node2.left) &&
+        treeEqual(node1.right, node2.right);
+}

utilities/treeMirror.js

@@ -0,0 +1,10 @@
+function treeMirror(node) {
+    if (!node) {
+        return;
+    }
+    let temp = node.left;
+    node.left = node.right;
+    node.right = temp;
+    treeMirror(node.left);
+    treeMirror(node.right);
+}

utilities/tree_equal.py

@@ -0,0 +1,8 @@
+def tree_equal(node1, node2):
+    if not node1 and not node2:
+        return True
+    if not node1 or not node2:
+        return False
+    return node1.val == node2.val and \
+        tree_equal(node1.left, node2.left) and \
+        tree_equal(node1.right, node2.right)

utilities/tree_mirror.py

@@ -0,0 +1,6 @@
+def tree_mirror(node):
+    if not node:
+        return
+    node.left, node.right = node.right, node.left
+    tree_mirror(node.left)
+    tree_mirror(node.right)

utilities/tree_traversal.py

@@ -0,0 +1,62 @@
+# Various iterative ways of traversing a tree.
+def inorder_traversal(root):
+    """
+    :type root: TreeNode
+    :rtype: List[int]
+    """
+    if not root:
+      return []
+    result = []
+    stack = [root]
+    while len(stack) > 0:
+        curr_node = stack.pop()
+        if curr_node.left:
+            stack.append(curr_node)
+            stack.append(curr_node.left)
+            curr_node.left = None
+        else:
+            result.append(curr_node.val)
+            if curr_node.right:
+                stack.append(curr_node.right)
+    return result
+
+def preorder_traversal(root):
+    """
+    :type root: TreeNode
+    :rtype: List[int]
+    """
+    if not root:
+        return []
+    result = []
+    stack = [root]
+    while len(stack) > 0:
+        curr_node = stack.pop()
+        result.append(curr_node.val)
+        if curr_node.right:
+            stack.append(curr_node.right)
+        if curr_node.left:
+            stack.append(curr_node.left)
+    return result
+
+def postorder_traversal(root):
+    """
+    :type root: TreeNode
+    :rtype: List[int]
+    """
+    if not root:
+        return []
+    result = []
+    stack = [root]
+    while len(stack) > 0:
+        curr_node = stack.pop()
+        if curr_node.left:
+            stack.append(curr_node)
+            stack.append(curr_node.left)
+            curr_node.left = None
+        elif curr_node.right:
+            stack.append(curr_node)
+            stack.append(curr_node.right)
+            curr_node.right = None
+        else:
+            result.append(curr_node.val)
+    return result

utilities/trie.py

@@ -0,0 +1,80 @@
+class Trie(object):
+    def __init__(self):
+        """
+        Initialize your data structure here.
+        """
+        self.d = {}
+
+    def insert(self, word):
+        """
+        Inserts a word into the trie.
+        :type word: str
+        :rtype: void
+        """
+        curr = self.d
+        for char in word:
+            if char not in curr:
+                curr[char] = {}
+            curr = curr[char]
+        curr['#'] = {} # Using an empty dict rather than a boolean value makes recursive traversal easier.
+
+    def search(self, word):
+        """
+        Returns if the word is in the trie.
+        :type word: str
+        :rtype: bool
+        """
+        curr = self.d
+        for char in word:
+            if char in curr:
+                curr = curr[char]
+            else:
+                return False
+        return '#' in curr
+
+    def startsWith(self, prefix):
+        """
+        Returns if there is any word in the trie that starts with the given prefix.
+        :type prefix: str
+        :rtype: bool
+        """
+        curr = self.d
+        for char in prefix:
+            if char in curr:
+                curr = curr[char]
+            else:
+                return False
+        return True
+
+    def searchRegex(self, word):
+        """
+        Returns if the word is in the data structure. A word could contain the dot character '.' to represent any one letter.
+        :type word: str
+        :rtype: bool
+        """
+        def traverse(node, index):
+            if len(word) == index:
+                return '#' in node
+            char = word[index]
+            if char == '.':
+                for key in node.keys():
+                    if traverse(node[key], index+1):
+                        return True
+                return False
+            else:
+                if char not in node:
+                    return False
+                return traverse(node[char], index + 1)
+        return traverse(self.d, 0)
+
+# Example
+trie = Trie()
+trie.insert('hello')
+print(trie.search('hello') == True)
+print(trie.startsWith('hello') == True)
+print(trie.startsWith('hel') == True)
+print(trie.search('world') == False)
+print(trie.startsWith('wor') == False)
+print(trie.searchRegex('..llo') == True)
+print(trie.searchRegex('..llx') == False)
+print(trie.searchRegex('..') == False)