/in/foo.cc: In function 'std::__cxx11::string cf(std::__cxx11::string)':
/in/foo.cc:10:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(a.size()<=len||a<s)
        ~~~~~~~~^~~~~
/in/foo.cc:14:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<=a.size();i++)
               ~^~~~~~~~~~
/in/foo.cc:26:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(a.size()==len)
      ~~~~~~~~^~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:115:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<s.size();i++)
               ~^~~~~~~~~