/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:19:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<G[x].size();i++){
               ~^~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:41:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int j=0;j<G[i].size();j++){
               ~^~~~~~~~~~~~