SICP 2.27 deep_reverse (迭代+递归)

练习2.27：现有 (list (list 1 2) (list 3 4 5)) 将其翻转构造成 (list (list 5 4 3) (list 2 1))
scheme代码：

**(define (deep_reverse tree)    (define (deep_reverse_it tree re)      (cond ((null? tree)             re)            (else              (deep_reverse_it (cdr tree)                (cons (if (pair? (car tree))                          (deep_reverse (car tree))                          (car tree))                  re)))))    (deep_reverse_it tree '()))(deep_reverse (list (list 1 2) (list 3 4 5)))((5 4 3) (2 1))**

这个程序很巧妙地运用了迭代和递归思想，在迭代过程 deep_reverse_it 中运用了递归调用了deep_reverse_it外部的函数。