foo.cc: In function 'void shuru()':
foo.cc:18:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:36:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:54:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<ss.length();i++)
              ~^~~~~~~~~~~~
foo.cc:71:15: 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。