/in/foo.cc: In function 'int ls(std::__cxx11::string, int)':
/in/foo.cc:5:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(s.size()<k)
     ~~~~~~~~^~
/in/foo.cc:11:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(si<s.size()&&f[s[si]]>=k)
        ~~^~~~~~~~~
/in/foo.cc:13:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(si==s.size())
     ~~^~~~~~~~~~