foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
foo.cc:18:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   18 |  for(int i=0;i<M.size();i++)
      |              ~^~~~~~~~~
foo.cc:20:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |   for(int j=0;j<M[0].size();j++)
      |               ~^~~~~~~~~~~~

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