foo.cc: In function 'int main()':
foo.cc:14:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(;i<=s.length();i++)
       ~^~~~~~~~~~~~
foo.cc:10:9: warning: 'ei' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int si,ei;
         ^~
foo.cc:10:6: warning: 'si' may be used uninitialized in this function [-Wmaybe-uninitialized]
  int si,ei;
      ^~

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