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#lang racket
(define/contract (find-error-nums nums)
(-> (listof exact-integer?) (listof exact-integer?))
(define nums-set (list->set nums))
(define range-set (apply set (range 1 (+ 2 (set-count nums-set)))))
(define missing-num (first (set->list (set-subtract range-set nums-set))))
(define necessary-num
(if (set-member? nums-set (- missing-num 1))
(+ missing-num 1)
(- missing-num 1)))
(list missing-num necessary-num))
(find-error-nums '(1 2 2 4))
(define fact-stream
(letrec ([f (lambda (x y)
(cond
[(zero? (modulo (- y 1) 3)) (cons (* 3 x) (lambda() (f (* x
y) (+ y 1))))]
[else (cons x (lambda() (f (* x y) (+ y 1))))])
[else (cons x (lambda() (f (* x y) (+ y 1))))]
(lambda () (f 1 2))
)])))
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