SICP 习题2.41 triple 三元组

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本文介绍了一种使用Scheme语言实现的算法，该算法能够找出指定范围内所有整数三元组，这些三元组的元素之和等于给定的目标值。通过列举区间内的整数并组合成三元组，利用过滤条件筛选出符合条件的三元组。

非常朴素的想法，找出所有的三元组，然后判定三元组的和是否与s相等

(define (accumulate op initial sequence)
  (if (null? sequence)
    initial
    (op (car sequence)
        (accumulate op initial (cdr sequence)))))

(define (enumerate-interval low high)
  (if (> low high) '() (cons low (enumerate-interval (+ low 1) high))))

(define (fliter predicate sequence)
  (cond ((null? sequence) '())
        ((predicate (car sequence))
         (cons (car sequence) (fliter predicate (cdr sequence))))
        (else (fliter predicate (cdr sequence)))))

(define (flatmap proc seq)
  (accumulate append '() (map proc seq)))

(define (remove item sequence)
  (fliter (lambda (x) (not (= item x))) sequence))

(define (triple n)
  (flatmap (lambda (i)
             (flatmap (lambda (j)
                        (map (lambda (k) (list i j k))
                             (remove i (remove j (enumerate-interval 1 n)))))
                      (remove i (enumerate-interval 1 n))))
           (enumerate-interval 1 n)))

(define (n-triple-s n s)
  (fliter (lambda (x)
            (= (+ (car x) (cadr x) (caddr x)) s))
          (triple n)))

(newline)
(display (n-triple-s 8 10))