foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
foo.cc:9:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    9 |         for(int i=0; i<M.size(); i++)
      |                      ~^~~~~~~~~
foo.cc:11:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   11 |                 for(int j=0; j<M[i].size(); j++)
      |                              ~^~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务