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

[Hydro](https://hydro.ac)提供评测服务