[SICP] [CH 2.2] 层次性数据和闭包性质

2.2.2 层次性结构

计算叶节点

(leaves tree) = (leaves (car tree)) + (leaves (cdr tree)),

(leaves leave) = 1;

; 计算tree中叶节点数量(define (leaves tree)
  (cond ((null? tree) 0)
        ((not (pair? tree)) 1)
        (else (+ (leaves (car tree))
                 (leaves (cdr tree))))))

; 计算叶节点的测试代码(define x (cons (list 1 2) (list 3 4))) ; ((1 2) 3 4))
(length x)             ; 3
(leaves x)             ; 4
(list x x)             ; (((1 2) 3 4) ((1 2) 3 4))
(length (list x x))    ; 2
(leaves (list x x))    ; 8

Q 2.28

题目：给出过程定义fringe, 以一个树为参数, 返回一个由这棵树中所有叶节点组成的表, 从左到右排列。

思路：用到了一个全局变量记录结果，遍历树，将所有叶节点append到结果上。

(define (fringe tree)
  (define (iter tree result)
    (cond ((null? tree) result)                               ; 当前结点为空时，返回result
          ((leaf? tree) (append result (list tree)))          ; 当前节点为叶时，封装为list，append至结果
          (else (iter (cdr tree) (iter (car tree) result))))) ; 当前节点为根结点时，将右结点append到左结点上
  (iter tree null))