foo.cc: In function 'int main()':
foo.cc:17:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   17 |     for (int i = 0; i < s.size(); i++) b[s[i] - 'a']++;
      |                     ~~^~~~~~~~~~

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