电话号码的字母组合(循环代替递归）

题目：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/submissions/ 参考：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/solution/zhi-xing-yong-shi-0mschao-guo-100yong-hu-by-su-ren/

class Solution {
public:
    vector letterCombinations(string digits) {
        /*
        *循环代替递归，res存储用了前k个字符时对应的字母集合
        *
        */
        int len = digits.size();
        vector ve = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        vector res;
        if(!len) return res;//特判空串
        res.push_back("");
        for(int k = 0,x,sz;k