/in/foo.cc: In function 'int dfs(const std::vector<int>&)':
/in/foo.cc:29:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int k = 0; k < G[u].size(); k++) B.push_back(G[u][k]);
                 ~~^~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:48:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int i = 0; i < G[0].size(); i++) 
                 ~~^~~~~~~~~~~~~