(define (queens board-size)
(define (queen-cols k)
(if (= k 0)
(list empty-board)
(filter
(lambda (positions) (safe? k positions))
(flatmap
(lambda (rest-of-queens)
(map (lambda (new-row)
(adjoin-position new-row k rest-of-queens))
(enumerate-interval 1 board-size)))
(queen-cols (- k 1))))))
(queen-cols board-size))
(define empty-board nil)
(define (adjoin-position new-row k rest-of-queens)
(cons (list new-row k) rest-of-queens))
(define (safe? k positions)
(define (iter lis)
(cond ((null? lis) #t)
((= (car (car positions)) (car (car lis))) #f)
((= (- (cadar positions) (cadar lis)) (abs (- (car (car positions)) (car (car lis))))) #f)
(else (iter (cdr lis)))))
(iter (cdr positions)))
gosh> (queens 5)
(((4 5) (2 4) (5 3) (3 2) (1 1)) ((3 5) (5 4) (2 3) (4 2) (1 1)) ((5 5) (3 4) (1 3) (4 2) (2 1)) ((4 5) (1 4) (3 3) (5 2) (2 1)) ((5 5) (2 4) (4 3) (1 2) (3 1)) ((1 5) (4 4) (2 3) (5 2) (3 1)) ((2 5) (5 4) (3 3) (1 2) (4 1)) ((1 5) (3 4) (5 3) (2 2) (4 1)) ((3 5) (1 4) (4 3) (2 2) (5 1)) ((2 5) (4 4) (1 3) (3 2) (5 1)))
