foo.cc: In function 'void shuru()':
foo.cc:16:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:32:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:48:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:64:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~

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