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
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]].