foo.cc: In function 'int main()':
foo.cc:14:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int i = 0; i < arr.size(); i++)
                 ~~^~~~~~~~~~~~
foo.cc:24:12: 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。