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.
Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
给定一个数字字符串，返回数字可能表示的所有可能的字母组合。
数字到字母的映射（就像电话按钮一样）在下面给出。
转载于:https://www.cnblogs.com/WegYcx/p/7793925.html
