2.59

练习 2.59 请为采用未排序表的集合实现定义union-set操作。

 
(define (element-of-set? x set)
  (cond ((null? set) #f)
        ((equal? x (car set)) #t)
        (else (element-of-set? x (cdr set)))
        )
  )
​
(element-of-set? 1 (list 1 2 3 4))
​x
 
true

 
(define (union-set set1 set2)
  (cond
   ((null? set1) set2)
   ((null? set2) set1)
   ((element-of-set? (car set1) set2) (union-set (cdr set1) set2))
   (else (cons (car set1) (union-set (cdr set1) set2)))
   )
  )
​
; 期待得到 (1 2 3 4 8)
(union-set (list 1 2 3 4) (list 1 2 3 8))
 
(4 1 2 3 8)