/in/foo.cc: In function 'int main()':
/in/foo.cc:18:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<G[i].size();j++){
               ~^~~~~~~~~~~~
/in/foo.cc:14:22: warning: array subscript is below array bounds [-Warray-bounds]
       G[-1].push_back(z);
       ~~~~~~~~~~~~~~~^~~