foo.cc: In function 'int main(int, const char**)':
foo.cc:12:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0, j = 1; i < ISBN.size(); ++i){
                        ~~^~~~~~~~~~~~~
foo.cc:26:2: warning: 'num' may be used uninitialized in this function [-Wmaybe-uninitialized]
  if(num == should) printf("Right");
  ^~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。