foo.cc: In member function 'std::vector<std::vector<long long int> > Solution::multiply(std::vector<std::vector<long long int> >, std::vector<std::vector<long long int> >)':
foo.cc:73:31: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<long long int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   73 |             for (int i = 0; i < a.size(); i++) {
      |                             ~~^~~~~~~~~~
foo.cc:74:35: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   74 |                 for (int j = 0; j < b[0].size(); j++) {
      |                                 ~~^~~~~~~~~~~~~
foo.cc:75:39: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   75 |                     for (int l = 0; l < a[0].size(); l++) {
      |                                     ~~^~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务