/in/foo.cc: In function 'int countSubstrings(const string&, int)':
/in/foo.cc:8:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.length(); i++) {
                     ~~^~~~~~~~~~~~
/in/foo.cc:10:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = i; j < s.length(); j++) {
                         ~~^~~~~~~~~~~~
/in/foo.cc:12:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (seen.size() == k) {
                 ~~~~~~~~~~~~^~~~
/in/foo.cc:14:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             } else if (seen.size() > k) {
                        ~~~~~~~~~~~~^~~