foo.cc: In function 'int main()':
foo.cc:11:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   11 |  for (i = 0; i < s.size(); i++)
      |              ~~^~~~~~~~~~
foo.cc:20:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |  for (i = idx + 1; i < s.size(); i++)
      |                    ~~^~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务