foo.cc: In function 'std::string tran_zs(std::string)':
foo.cc:16:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   16 |         for (int i = 0; i < M.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc: In function 'std::string jiafa(std::string, std::string, int)':
foo.cc:28:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   28 |         for (int i = 0; i < M.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc:33:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   33 |         for (int i = 0; i < M.size(); i++)
      |                         ~~^~~~~~~~~~
foo.cc: In function 'bool check(std::string)':
foo.cc:46:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   46 |         for (int i = 0; i < M.size() / 2; i++)
      |                         ~~^~~~~~~~~~~~~~

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