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:9:23: warning: array subscript has type 'char' [-Wchar-subscripts]
    9 |                 b[a[i]]++;
      |                       ^
foo.cc:10:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |         for(int i=0;i<a.size();i++)
      |                     ~^~~~~~~~~
foo.cc:11:26: warning: array subscript has type 'char' [-Wchar-subscripts]
   11 |                 if(b[a[i]]==1)
      |                          ^

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