sicp每日一题[2.62]

Exercise 2.62

Give a (n) implementation of union-set for sets represented as ordered lists.

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这道题难度也不大，思路是依次从两个集合中取第一个元素，比较他们的大小，把小的那个和剩下所有元素连接的结果连接起来。由于两个集合中每个元素都只需要取出一次，所以时间复杂度应该是 O(n)。

(define (union-set set1 set2)
  (cond ((null? set1) set2)
        ((null? set2) set1)
        ((let ((s1 (car set1))
               (s2 (car set2)))
           (if (<= s1 s2)
               (cons s1 (union-set (cdr set1) set2))
               (cons s2 (union-set set1 (cdr set2))))))))

(define set1 (list 1 3 5 7 9))
(define set2 (list 2 4 6 8 10))

(union-set set1 set2)

; 结果如下所示
'(1 2 3 4 5 6 7 8 9 10)