Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example:
Input: "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
17. Letter Combinations of a Phone Number 17.电话号码的字母组合
原文：https://www.cnblogs.com/immiao0319/p/13594142.html