/in/foo.cc: In function 'int main()':
/in/foo.cc:11:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < s.length(); i++) {
              ~~^~~~~~~~~~~~
/in/foo.cc:14:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (j = i + 1; j < s.length(); j++) {
                   ~~^~~~~~~~~~~~
/in/foo.cc:27:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if ((j == s.length() && k == -1) && !success)
        ~~^~~~~~~~~~~~~