foo.cc: In function 'int main()':
foo.cc:10:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0,b=0;i<s.length();i++)
                  ~^~~~~~~~~~~
foo.cc:22:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<s.length()-1;i++) cout<<s[i];
               ~^~~~~~~~~~~~~

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