foo.cc: In function 'void Replace(std::__cxx11::string&, std::__cxx11::string, std::__cxx11::string)':
foo.cc:18:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(j = 0; j < from_str.length();)  //from_str中的第j个字符
              ~~^~~~~~~~~~~~~~~~~~~
foo.cc:43:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int k = 0; k < to_str.length();)  //to_str中的第k个字符
                   ~~^~~~~~~~~~~~~~~~~

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