class Solution {public:    string longestCommonPrefix(vector
    
     & strs) {        if (strs.empty()) return "";        string res = "";        for (int j = 0; j < strs[0].size(); ++j) {            char c = strs[0][j];            for (int i = 1; i < strs.size(); ++i) {                if (j >= strs[i].size() || strs[i][j] != c) {                    return res;                }            }            res.push_back(c);        }        return res;    }};