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 < arr.size(); i++)
                     ~~^~~~~~~~~~~~
foo.cc:24:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         while (loc >= arr.size())
                ~~~~^~~~~~~~~~~~~

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