sicp 2.66

sicp_2_66

(define (entry tree) (car tree))
(define (left-branch tree) (cadr tree))
(define (right-branch tree) (caddr tree))
(define (make-tree entry left right)
(list entry left right))


(define tree1 (make-tree 7
                           (make-tree 3
                                      (make-tree 1 '() '())
                                      (make-tree 5 '() '()))
                           (make-tree 9
                                      '()
                                      (make-tree 10 '() '()))))
 
(define tree2  (make-tree 3
                           (make-tree 1 '() '())
                           (make-tree 7
                                      (make-tree 5 '() '())
                                      (make-tree 9
                                                 '()
                                                 (make-tree 24 '() '())))))

(define (lookup x set)
(cond ((null? set) #f)
((= x (entry set)) (entry set))
((< x (entry set))
(lookup x (left-branch set)))
((> x (entry set))
(lookup x (right-branch set)))))

(lookup 3 tree1)
(lookup 55 tree1)