foo.cc: In function 'bool Trans(std::__cxx11::string&, std::__cxx11::string&)':
foo.cc:21:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (i = 0; i<str.size(); i++) {
                 ~^~~~~~~~~~~

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