foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
foo.cc:25:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   25 |  for(int i=0; i<M.size(); i++)
      |               ~^~~~~~~~~
foo.cc:27:17: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |   for(int j=0; j<M[i].size(); j++)
      |                ~^~~~~~~~~~~~
foo.cc: In function 'void SortRow(std::vector<std::vector<int> >&)':
foo.cc:36:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   36 |  for(int k=0; k<M.size(); k++)//要对M.size行进行排序
      |               ~^~~~~~~~~
foo.cc: In function 'int main(int, char**)':
foo.cc:10:9: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
   10 |  freopen("data.txt", "r", stdin);
      |  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务