decode a string

string fun (int &idx, string &str)
    {
        string res = "";
        string decoded_string = "";

        while (idx < str.size())
        {

            if (isdigit(str[idx]))
            {

                string num ="";
                while ( idx < str.size() && isdigit(str[idx] ) )
                       num += str[idx++];
                int t = stoi (num);

                idx++;
                decoded_string = fun (idx,str);

                for (int ttimes =  1 ; ttimes <= t ; ttimes++)
                    res += decoded_string;

            }
            else if (isalpha(str[idx]) )
                res+= str[idx];

            else if (str[idx] == ']')
                return res;

            idx++;

        }
         return res;
    }
    string decodeString(string s) {
       int idx = 0 ;
        return fun (idx ,s);


    }