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.
Although the above answer is in lexicographical order, your answer could be in any order you want.
Given "23"
Return["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
[LintCode] Letter Combinations of a Phone Number 电话号码的字母组合
原文：http://www.cnblogs.com/grandyang/p/5690104.html