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:8:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    8 |   for (int j = 0; j < M[i].size(); j++)
      |                   ~~^~~~~~~~~~~~~
foo.cc: In function 'void Sortbyrow(std::vector<std::vector<int> >&)':
foo.cc:15:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   15 |  for (int k = 0; k < M.size(); k++)
      |                  ~~^~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务