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| ;; From scheme wiki | |
| (define (contains-cycle? lst) | |
| (define (safe-cdr l) | |
| (if (pair? l) | |
| (cdr l) | |
| '())) | |
| (define (iter a b) | |
| (cond | |
| ((not (pair? a)) #f) | |
| ((not (pair? b)) #f) | |
| ((eq? a b) #t) | |
| ((eq? a (safe-cdr b)) #t) | |
| (else | |
| (iter (safe-cdr a) (safe-cdr (safe-cdr b)))))) | |
| (iter (safe-cdr lst) (safe-cdr (safe-cdr lst)))) | |
| (define x '(1 2 3 4 5 6 7 8)) | |
| (define y '(1 2 3 4 5 6 7 8)) | |
| (set-cdr! (cdddr (cddddr y)) (cdddr y)) | |
| (define z '(1)) | |
| (contains-cycle? x) | |
| (contains-cycle? y) |