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Finding duplicates
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| #lang racket | |
| (define (find-duplicates elts) | |
| (define ht (make-hash)) | |
| (for/fold ([dups '()]) ([x (in-list elts)]) | |
| (hash-set! ht x (add1 (hash-ref ht x 0))) | |
| (cond | |
| [(= (hash-ref ht x) 2) | |
| (cons x dups)] | |
| [else | |
| dups]))) | |
| (find-duplicates '(1 2 2 3 1 4 5 4 4 4)) |
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