17 Letter Combinations of a Phone Number（medium）

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。