1.6 String Compression

Just a simple string problem.

    string compress(string str){
        string res = "";
        int cnt = 1;
        for (int i = 0; i<str.size(); ++i) {
            if (res.size() >= str.size()) return str;
            else if(str[i] == str[i+1] && i+1<str.length()) ++cnt;
            else{
                res += (str[i] + to_string(cnt));
                cnt = 1;
            }
        }
        return res;
    }