SICP 2.62

楽しいのぅ。

(define (union-set s1 s2)
  (cond ((null? s1) s2)
        ((null? s2) s1)
        ((= (car s1) (car s2)) (cons (car s1)
                                     (union-set (cdr s1) (cdr s2))))
        ((< (car s1) (car s2)) (cons (car s1)
                                     (union-set (cdr s1) s2)))
        (else (cons (car s2) (union-set s1 (cdr s2))))))

; test
(union-set '(1 2 3 4) '(1 3 5))
(union-set '(1 3 5) '(2 4 6))
(union-set '(1 3 5) '(1 3 5))