Letter Combinations Of A Phone Number
Problem
Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.
Example
Given 23
Return ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
Note
Although the above answer is in lexicographical order, your answer could be in any order you want.