Indicate problems that are LeetCode premium only (#12)

algorithms/README.md

@@ -170,11 +170,11 @@ def traverse(matrix):
 
 - [Clone Graph](https://leetcode.com/problems/clone-graph/)
 - [Course Schedule](https://leetcode.com/problems/course-schedule/)
-- [Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)
+- [Alien Dictionary (Leetcode Premium)](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/)
+- [Graph Valid Tree (Leetcode Premium)](https://leetcode.com/problems/graph-valid-tree/)
+- [Number of Connected Components in an Undirected Graph (Leetcode Premium)](https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/)
 - [Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/)
 
 ## Interval
@@ -204,7 +204,7 @@ def merge_overlapping_intervals(a, b):
 
 - [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/)
+- [Meeting Rooms (Leetcode Premium)](https://leetcode.com/problems/meeting-rooms/) and [Meeting Rooms II (Leetcode Premium)](https://leetcode.com/problems/meeting-rooms-ii/)
 - [Non-overlapping Intervals](https://leetcode.com/problems/non-overlapping-intervals/)
 
 ## Linked List
@@ -297,7 +297,7 @@ transposed_matrix = zip(*matrix)
 #### 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/)
+- [Strobogrammatic Number II (Leetcode Premium)](https://leetcode.com/problems/strobogrammatic-number-ii/)
 
 ## String
 
@@ -350,7 +350,7 @@ for c in set(word):
 - [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/)
+- [Encode and Decode Strings (Leetcode Premium)](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)