Exercise 2.66
Implement the lookup procedure for the case where the set of records is structured as a binary tree, ordered by the numerical values of the keys.
这道题还是挺简单的，由于是有序的二叉树，所以可以先比较根与要查找的数字的大小，如果相等就查到了；如果根小于要查找的数，则递归调用判断右子树中是否包含这个数; 如果根大于要查找的数，则去左子树去查即可。
(define (lookup given-key set-of-records)
(if (null? set-of-records)
false
(let ((root (entry set-of-records)))
(cond ((= given-key root) true)
((< given-key root) (lookup given-key (left-branch set-of-records)))
(else (lookup given-key (right-branch set-of-records)))))))
(define test (list->tree (list 1 2 3 4 5 6 7 8 9)))
(lookup 7 test)
(lookup 10 test)
; 结果如下
#t
#f