foo.cc: In function ‘int countSubstrings(const std::string&, int)’:
foo.cc:8:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |     for (int i = 0; i < s.length(); i++) {
      |                     ~~^~~~~~~~~~~~
foo.cc:10:27: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   10 |         for (int j = i; j < s.length(); j++) {
      |                         ~~^~~~~~~~~~~~
foo.cc:12:29: warning: comparison of integer expressions of different signedness: ‘std::unordered_set<char>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
   12 |             if (seen.size() == k) {
      |                 ~~~~~~~~~~~~^~~~
foo.cc:14:36: warning: comparison of integer expressions of different signedness: ‘std::unordered_set<char>::size_type’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
   14 |             } else if (seen.size() > k) {
      |                        ~~~~~~~~~~~~^~~