foo.cc: In function 'std::__cxx11::string trans(const string&, int, int)':
foo.cc:24:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int k=0; k < orginal[j].length();k++)
                   ~~^~~~~~~~~~~~~~~~~~~~~
foo.cc: In function 'void bfs()':
foo.cc:54:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i=0;i < u.str.length();i++)//枚举当前串所有可能位置
                      ~~^~~~~~~~~~~~~~~~

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