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<a.size();i++)
      |                     ~^~~~~~~~~
foo.cc:10:31: warning: array subscript has type 'char' [-Wchar-subscripts]
   10 |                         s[a[i]]++;
      |                               ^

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