foo.cc: In function 'int main()':
foo.cc:15:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   15 |         for(int i=0;i<s.size();i++) sum[s[i]-'a']++;
      |                     ~^~~~~~~~~

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