/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:24:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<G[x].size();++i)
                 ~^~~~~~~~~~~~
/in/foo.cc: In constructor 'node::node()':
/in/foo.cc:84:38: warning: passing NULL to non-pointer argument 2 of 'void* memset(void*, int, size_t)' [-Wconversion-null]
         memset(head,NULL,sizeof(head));
                                      ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:101:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=0;j<G[i].size();++j)
                     ~^~~~~~~~~~~~
/in/foo.cc:112:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=0;j<G[i].size();++j)
                     ~^~~~~~~~~~~~