foo.cpp: In function 'void tarjan(int)':
foo.cpp:38:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < G[u].size(); i += 1){
                  ~~^~~~~~~~~~~~~
foo.cpp: In function 'int main()':
foo.cpp:84:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 0; j < G[i].size(); j += 1){
                   ~~^~~~~~~~~~~~~