foo.cc: In function ‘int main()’:
foo.cc:7:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    7 |     for (int i = 1; i < s.size(); i ++)
      |                     ~~^~~~~~~~~~