/in/foo.cc: In function 'int main()':
/in/foo.cc:27:22: warning: array subscript has type 'char' [-Wchar-subscripts]
   27 |    for(char c:s1) a1[c]++;
      |                      ^
/in/foo.cc:28:22: warning: array subscript has type 'char' [-Wchar-subscripts]
   28 |    for(char c:s2) a1[c]--;
      |                      ^