Exercise 3.13. Consider the following make-cycle procedure, which uses the last-pair procedure defined in exercise 3.12:
(define (make-cycle x)
(set-cdr! (last-pair x) x)
x)
Draw a box-and-pointer diagram that shows the structure z created by
(define z (make-cycle (list 'a 'b 'c)))
What happens if we try to compute (last-pair z)?
make-cycle 会把链表最后一个盒子的 cdr 指针指向第一个盒子，于是 z 就变成了
z +-------------------------------+
| | |
| | |
| v |
| +---+---+ +---+---+ +---+-+-+
+->| | +---->| | +---->| | |
+-+-+---+ +-+-+---+ +-+-+---+
| | |
| | |
v v v
+---+ +---+ +---+
| a | | b | | c |
+---+ +---+ +---+试图去计算 z 的最后一个 pair 会进入死循环。