2.17
(define (last-pair lst)
(if (null? (cdr lst))
(cons (car lst) ())
(last-pair (cdr lst))))
2.18
(define (reverse lst)
(define (iter lst-o lst-d)
(if (null? lst-o)
lst-d
(iter (cdr lst-o) (cons (car lst-o) lst-d))))
(iter lst null))
2.20
(define (same-parity x . lst)
(define (filter lst ok?)
(if (null? lst)
()
(if (ok? (car lst))
(cons (car lst) (filter (cdr lst) ok?))
(filter (cdr lst) ok?))))
(if (even? x)
(cons x (filter lst (lambda(x) (= 0 (remainder x 2)))))
(cons x (filter lst (lambda(x) (= 1 (remainder x 2)))))))
2.21
(define (square-list- items)
(if (null? items)
()
(cons (* (car items) (car items))
(square-list- (cdr items)))))
(define (square-list items)
(map (lambda(x) (* x x)) items))
2.22
第一种每次取出首元素平方后前插到新表，象reverse过程类似，所以是反的。
第二种只不过是把新表前插到元素前，得到的甚至不是一个list，而是
((((() . 1) . 4) . 9) . 16)
2.23
(define (for-each proc items)
(if (not (null? items))
((lambda() (proc (car items))
(for-each proc (cdr items))))))