leetcode --电话号码的字符组合（地平线面试题）

1. 题目描述
   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.

每一种可能情况下有有多种可能，要求列出所有可能情况或者找最优的情况，递归是最简单的。这里的n中可能情况，每种可能下又有m中，就像一棵树一样，十分类似走迷宫的情况*（4中可能）。有的时候还不知道树的层数，这种就必须递归了。

class Solution {
public:
	vector<string> letterCombinations(string digits) {
		vector<string> codes = { "","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz" };
		vector<string> ret;

		if (digits.empty())
			return ret;