foo.cc: In function 'int main()':
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++)
              ~^~~~~~~~~~~
foo.cc:14:6: warning: unused variable 'bn' [-Wunused-variable]
  int bn=b.length();
      ^~

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