foo.cc: In function 'int main()':
foo.cc:14:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.size(); i++)
                     ~~^~~~~~~~~~
foo.cc:22:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < qs.size(); i++)
                     ~~^~~~~~~~~~~
foo.cc:26:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = i + 1; j < qs.size();j++)
                             ~~^~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。