foo.cc: In function ‘bool isLuckyNumber(long long int)’:
foo.cc:10:23: 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 i = 0; i < numStr.length(); ++i)
      |                     ~~^~~~~~~~~~~~~~~~~