foo.cc: In function ‘int main()’:
foo.cc:8:22: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    8 |         for(int i=0;i<s.size()-1;i++)
      |                     ~^~~~~~~~~~~
foo.cc:9:25: warning: array subscript has type ‘char’ [-Wchar-subscripts]
    9 |                 cnt[s[i]]++;
      |                         ^