foo.cc: In function 'int main()':
foo.cc:19:14: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   19 |     for(i=0;i<s.length();i++)
      |             ~^~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务