/in/foo.cc: In function 'int main()':
/in/foo.cc:30:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   30 |         for (int i = 0; i < G[u].size(); i++) {
      |                         ~~^~~~~~~~~~~~~