Scheme N皇后

(define (range n)
    (define (recur n)
        (if (= n -1)
            '()
            (cons n (recur (- n 1)))))
    (recur (- n 1)))

(define (flatten a)
    (if (null? a)
        '()
        (append (car a) (flatten (cdr a)))))

(define (safe? x y sln)
    (if (null? sln)
        #t
        (let ((px (car (car sln))) (py (cadr (car sln))))
            (if (or (= y py) (= (- py y) (- px x)) (= (- py y) (- x px)))
                #f 
                (safe? x y (cdr sln))))))

(define (nqueen n)
    (define (recur x)
        (if (= x -1)
            (list '())
            (flatten (map (lambda (y) (map (lambda (sln) (cons (list x y) sln)) (filter (lambda (sln) (safe? x y sln)) (recur (- x 1))))) (range n)))))
    (recur (- n 1)))

(define (pl a)
    (if (null? a)
        '()
        (begin (display (car a)) (display "\n") (pl (cdr a)))))

(pl (nqueen 4))

 流 版本:

#lang racket
(require racket/stream)

(define-syntax-rule (scons a b) (stream-cons a b))
(define-syntax-rule (scar a) (stream-first a))
(define-syntax-rule (scdr a) (stream-rest a))
(define-syntax-rule (smap f s) (stream-map f s))
(define-syntax-rule (sfilter f s) (stream-filter f s))
(define-syntax-rule (sreduce f i s) (stream-fold f i s))
(define-syntax-rule (snull? s) (stream-empty? s))
(define-syntax-rule (sappend s ...) (stream-append s ...))
(define-syntax-rule (sfor f s) (stream-for-each f s))

(define (sflatten a)
    (if (snull? a)
        empty-stream
        (sappend (scar a) (sflatten (scdr a)))))

(define (srange n)
    (let recur ((x 0))
        (if (= x n)
            empty-stream
            (scons x (recur (+ x 1))))))

(define (dis x)
  (begin (display x)(newline)))

(define (sd s)
  (sfor dis s))

(define (safe? x y sln)
    (if (snull? sln)
        #t
        (let ((px (scar (scar sln))) (py (scar (scdr (scar sln)))))
            (if (or (= y py) (= (- py y) (- px x)) (= (- py y) (- x px)))
                #f 
                (safe? x y (scdr sln))))))

(define (nqueen n)
    (define (recur x)
        (if (= x -1)
            (stream empty-stream)
            (sflatten (smap (lambda (y) (smap (lambda (sln) (scons (stream x y) sln)) (sfilter (lambda (sln) (safe? x y sln)) (recur (- x 1))))) (srange n)))))
    (recur (- n 1)))


(define ass (sflatten (stream (srange 4) (srange 5))))
;(define ass  (stream (srange 4) (srange 5)))
(sfor (lambda (x) (sfor (lambda (y) (sfor display y)(display ",")) x)(newline)) (nqueen 8))