P12 (**) 解码游程编码过的列表

问题描述

sash> (decode '((4 a) b (2 c) (2 a) d (4 e)))
sash> (a a a a b c c a a d e e e e)

解法

递归实现

'(E ... E)

'(E)

append

(define to-list
(lambda (ne)
(if (list? ne)
(let f ([ls '()] [n (car ne)])
(cond
[(zero? n) ls]
[else (f (cons (cadr ne) ls) (- n 1))]))
(list ne))))
(define decode
(lambda (ls)
(cond
[(null? ls) '()]
[else (append (to-list (car ls))
(decode (cdr ls)))])))

decode

reduce