/in/foo.cc: In function 'void colsort(std::vector<std::vector<int> >&)':
/in/foo.cc:17:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M[0].size(); i++) {
                  ~~^~~~~~~~~~~~~
/in/foo.cc:18:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0; j < M.size(); j++) {
                   ~~^~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:43:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++) {
                  ~~^~~~~~~~~~
/in/foo.cc:44:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0; j < M[i].size(); j++) {
                   ~~^~~~~~~~~~~~~
/in/foo.cc:48:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (i != M.size() - 1) {
       ~~^~~~~~~~~~~~~~~