foo.cc: In function 'std::__cxx11::string qth(std::__cxx11::string)':
foo.cc:14:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<n.size();i++)
              ~^~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:68:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<n.size();i++)
              ~^~~~~~~~~

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