/in/foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
/in/foo.cc:23:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++) {
                  ~~^~~~~~~~~~
/in/foo.cc:24:21: 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:32:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int k = 0; k < M.size(); k++) {
                  ~~^~~~~~~~~~