foo.cc: In function 'int Diff(std::__cxx11::string)':
foo.cc:8:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size(); i++)
                  ~~^~~~~~~~~~

自豪的采用jd5进行评测(github.com/masnn/jd5)