foo.cc: In function 'int main()':
foo.cc:154:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < answer.size(); i++) m = std::max(m, answer[i]);
                     ~~^~~~~~~~~~~~~~~

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