2.40

(define (unique-pairs n)  (flatmap (lambda (i)             (map (lambda (j) (list i j))                  (enumerate-interval 1 (- i 1))))           (enumerate-interval 1 n)))

2.41

(define (gentuple n s)  (filter (lambda (x)            (= (accumulate + 0 x) s))          (flatmap (lambda (i)                     (flatmap (lambda (j)                                (map (lambda (k)                                       (list i j k))                                     (enumerate-interval 1 (- j 1))))                              (enumerate-interval 1 (- i 1))))                   (enumerate-interval 1 n))))

2.42

行号列号从1开始。从右向左逐步构造棋盘

(define empty-board '())(define (adjoin-position new-row k rest-of-queens)  (cons new-row rest-of-queens))(define (safe? k positions)  (define (iter ex-row ex1 ex2 lst)    (if (null? lst)        #t        (if (or (= ex-row (car lst))                (= ex1 (car lst))                (= ex2 (car lst)))            #f            (iter ex-row (- ex1 1) (+ ex2 1) (cdr lst)))))  (let ((c (car positions)))    (iter c (- c 1) (+ c 1) (cdr positions))))

2.43

对于2.42中的程序，当board-size为n时，queens-cols调用1+n次；

对于Louis的程序：当k=1时，queens-cols会调用board-size次；当k=2时，queens-cols会调用board-size乘以当k=1时的次数；归纳得，queens-cols会调用board-size的board-size次方次。

于是Louis的程序的运行时间大约为2.42程序的board-size的board-size -1次方倍，

即：(* (expt board-size (- board-size 1)) T)