(define n-queens
  (lambda (n)
    (define try-next-solution
      (lambda (queens-so-far start-row in-column)
        ;(display (list queens-so-far start-row in-column)) (newline)
        (if (> in-column n)
            queens-so-far
            (if (<= start-row n)
                (if (safe? start-row in-column queens-so-far)
                    (try-next-solution (cons (cons start-row in-column) queens-so-far) 1 (+ in-column 1) )
                    (try-next-solution queens-so-far (+ start-row 1) in-column))
                (if (null? queens-so-far)
                    #f
                    (try-next-solution (cdr queens-so-far) (+ 1 (caar queens-so-far)) (cdar queens-so-far)))))))
    (try-next-solution '() 1 1)))

(define safe?
  (lambda (row col list)
    (cond
      ((null? list) #t)
      ((= row (caar list)) #f)
      ((same-diagonal? row col (caar list) (cdar list)) #f)
      ((same-left-d? row col (caar list) (cdar list)) #f)
      (else (safe? row col (cdr list))))))

(define same-diagonal?
  (lambda (r1 c1 r2 c2)
    (= (- c1 r1) (- c2 r2))))

(define same-left-d?
  (lambda (r1 c1 r2 c2)
    (= (- r1 r2) (- c2 c1))))