foo.cc: In function 'int hf(std::__cxx11::string)':
foo.cc:8:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for(register int i=0;i<st.size();++i){
                      ~^~~~~~~~~~

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