/in/foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
/in/foo.cc:19:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0; i<M.size(); i++)
               ~^~~~~~~~~
/in/foo.cc:21:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0; j<M[i].size(); j++)
                ~^~~~~~~~~~~~
/in/foo.cc: In function 'void SortRow(std::vector<std::vector<int> >&)':
/in/foo.cc:29:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int k=0; k<M.size(); k++)
               ~^~~~~~~~~