/in/foo.cc: In function 'int main()':
/in/foo.cc:21:30: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   21 |                 for(int j=0;j<a[i].size();j++)
      |                             ~^~~~~~~~~~~~