foo.cc: In function ‘bool spfa(long long int, long long int)’:
foo.cc:28:30: warning: comparison of integer expressions of different signedness: ‘long long int’ and ‘std::vector<Edge>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   28 |                 for(int i=0;i<G[u].size();i++){
      |                             ~^~~~~~~~~~~~