; find the difference
 
(define (sum xs) (apply + xs))
 
(define (diff1 xstr ystr)
  (let loop ((xs (sort (string->list xstr) char<?))
             (ys (sort (string->list ystr) char<?)))
    (cond ((null? xs) (car ys))
          ((char=? (car xs) (car ys))
            (loop (cdr xs) (cdr ys)))
          (else (car ys)))))
 
(display  (diff1 "abcdef" "bfxdeac")) (newline)
 
(define (diff2 xstr ystr)
  (let ((x (sum (map char->integer (string->list xstr))))
        (y (sum (map char->integer (string->list ystr)))))
    (integer->char (- y x))))
 
(display  (diff2 "abcdef" "bfxdeac")) (newline)

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