在C ++中复制字符时修改字符向量

Question

以下是我提交的

https://leetcode.com/problems/letter-combinations-of-a-phone-number

然而，我看到线上非常奇怪的行为

char c = map1[s[i]][j];
t.replace(i,1,1,c);

矢量map1 [s [i]]正在改变下面代码中上行执行的大小。如何通过值而不是引用来复制字符c。如果我在函数参数中删除了字符串的引用（意思是没有＆amp;），一切正常，但它不是正确的方法，因为按值复制字符串是昂贵的。

class Solution {
public:
    string s;
    vector<string> res;
    unordered_map<char, vector<char>> map1;
    void letterCombinations(string &t, int i){
        char c;
        if(s.size()==0)
            return;
        if(i==s.size())
            res.push_back(t);
        else{
            for(int j=0;j<map1[s[i]].size();j++){
                //cout<<"Starting:"<<map1[s[i]].size()<<endl;
                char c = map1[s[i]][j];
                t.replace(i,1,1,c);
                letterCombinations(t, i+1);
                //cout<<"Return:"<<map1[s[i]].size()<<endl;
            }
        }
    }
    vector<string> letterCombinations(string digits) {
        stringstream t;
        map1['1'] = vector<char>{};
        map1['0'] = vector<char>{};
        map1['2'] = vector<char>{'a','b','c'};
        map1['3'] = vector<char>{'d','e','f'};
        map1['4'] = vector<char>{'g','h','i'};
        map1['5'] = vector<char>{'j','k','l'};
        map1['6'] = vector<char>{'m','n','o'};
        map1['7'] = vector<char>{'p','q','r','s'};
        map1['8'] = vector<char>{'t','u','v'};
        map1['9'] = vector<char>{'w','x','y','z'};

        s=digits;
        letterCombinations(s,0);
        return res;
    }

};