longest common prefix

string longestCommonPrefix(vector<string> &strs) {
    if(strs.size() == 0) return "";
    string res = "";
    bool prefix = true;
    for(int i=0; i<strs[0].size(); i++){
        for(int j=1; j<strs.size(); j++){
            if(i >= strs[j].size() || strs[j][i] != strs[0][i]){
                prefix = false;
                break;
            }
        }
        if(prefix){//初始值i == 0时
            res.append(1,strs[0][i]);
        }
        else break;
    }
    
    return res;
}