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))