SICP习题 1.17 (用加法求乘积)

(define (double x)
  (+ x x))

(define (halve x)
  (/ x 2))

(define (mult a b)
  (cond ((= b 0) 0)
        ((even? b) (double (mult a (halve b))))
        ((odd? b) (+ a (mult a (- b 1))))))

(mult 12 12)

　　

(define (double x)
  (+ x x))

(define (halve x)
  (/ x 2))

(define (mult a b)
  (cond ((= b 0) 0)
        ((even? b) (mult (double a) (halve b)))
        ((odd? b) (+ a (mult a (- b 1))))))

(mult 12 12)