/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:18:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<G[u].size();i++)
              ~^~~~~~~~~~~~
/in/foo.cc: In function 'int dfs(int, int)':
/in/foo.cc:54:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<nG[u].size();i++)
              ~^~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:86:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<G[u].size();i++)
               ~^~~~~~~~~~~~