foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
foo.cc:24:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   24 |  for(int i=0; i<M.size(); i++)
      |               ~^~~~~~~~~
foo.cc:26:17: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   26 |   for(int j=0; j<M[i].size(); j++)
      |                ~^~~~~~~~~~~~
foo.cc: In function 'void SortRow(std::vector<std::vector<int> >&)':
foo.cc:35:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   35 |  for(int k=0; k<M.size(); k++)//要对M.size行进行排序
      |               ~^~~~~~~~~

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