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#lang racket
(define/contract (contains-duplicate nums)
(-> (listof exact-integer?) boolean?)
(define nums-hash (make-hash))
(define (check-next-number nums)
(cond [(empty? nums) #false]
[(hash-ref nums-hash (car nums) #false) #true]
[else (hash-set! nums-hash (car nums) #true)
(check-next-number (cdr nums))]))
(check-next-number nums))
(contains-duplicate '(1 2 3))
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