Ex 2.30

map 使用:

(define (square-tree t)
  (map (lambda (x) (if (pair? x) (square-tree x) (square x))) t))

map 不使用:

(define (square-tree t)
  (cond (null? t) t)
        ((not (pair? t)) (square t))
        (else (cons (square-tree (car t)) (square-tree (cdr t))))))