/in/foo.cc: In function 'int DFS(int, int)':
/in/foo.cc:27:15: 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 main()':
/in/foo.cc:76:9: warning: unused variable 'new_n' [-Wunused-variable]
     int new_n=0;
         ^~~~~
/in/foo.cc:50:12: warning: iteration 999 invokes undefined behavior [-Waggressive-loop-optimizations]
     pre2[i]=i;
     ~~~~~~~^~
/in/foo.cc:49:18: note: within this loop
     for(int i=1;i<=maxn;i++)
                 ~^~~~~~