Exercise 2.28 · 习题

Exercise 2.28: Write a procedure fringe
that takes as argument a tree (represented as a list) and returns a list whose
elements are all the leaves of the tree arranged in left-to-right order. For
example,

(define x
(list (list 1 2) (list 3 4)))

(fringe x)
(1 2 3 4)

(fringe (list x x))

(1 2 3 4 1 2 3 4)

练习 2.28：写一个过程 fringe，它以一棵树（用表表示）为参数，返回一个表，其元素是该树所有的叶子，按从左到右的顺序排列。例如，

(define x
(list (list 1 2) (list 3 4)))

(fringe x)
(1 2 3 4)

(fringe (list x x))
(1 2 3 4 1 2 3 4)