foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
foo.cc:6:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    6 |  for (int i = 0; i < M.size(); i++)
      |                  ~~^~~~~~~~~~
foo.cc:9:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    9 |   for (int j = 0; j < M[i].size(); j++)
      |                   ~~^~~~~~~~~~~~~

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