foo.cc: In function 'bool solve(ll)':
foo.cc:112:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=0; j<arv[i].size(); j++) {
                      ~^~~~~~~~~~~~~~

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