string commonPrefix(string s1, string s2){    if(s1 == "" || s2 == "") return "";    int k = 0, len1 = s1.length();    while(k < len1 && s1[k] == s2[k]) ++k;    return s1.substr(0, k);}    class Solution {public:    string longestCommonPrefix(vector<string> &strs) {        int len = strs.size();        if(len == 0) return "";        string s(strs[0]);        for(int i = 1; i < len; ++i){            if(s == "") return s;            s = commonPrefix(s, strs[i]);        }        return s;    }};