/in/foo.cc: In function 'std::__cxx11::string tran_zs(std::__cxx11::string)':
/in/foo.cc:16:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string jiafa(std::__cxx11::string, std::__cxx11::string, int)':
/in/foo.cc:28:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc:33:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc: In function 'bool check(std::__cxx11::string)':
/in/foo.cc:46:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size() / 2; i++)
                  ~~^~~~~~~~~~~~~~