; Determines whether a list has a cycle
(define (has-cycle? x)
  (define (iter fast-ptr slow-ptr)
    (cond ((null? fast-ptr) #f)
          ((null? (cdr fast-ptr)) #f)
          ((eq? fast-ptr slow-ptr) #t)
          (else (iter (cddr fast-ptr) (cdr slow-ptr)))))
  (if (null? x)
      #f
      (iter x (cdr x))))