foo.cc: In function ‘int main()’:
foo.cc:8:23: warning: comparison of integer expressions of different signedness: ‘char’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |         for(char i=0;i<s.size();i++)
      |                      ~^~~~~~~~~