/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i<a[u].size(); i++) {
                  ~^~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:50:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=0; j<a[i].size(); j++) {
                      ~^~~~~~~~~~~~