foo.cc: In function 'int main()':
foo.cc:30:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < G[u].size(); i++) {
                         ~~^~~~~~~~~~~~~

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