P02 (*) 获取列表的倒数第二个元素

sash> (last_but_one '(a b c d))
sash> c

#t

(define last_but_one
(lambda (ls)
(if (null? ls)
(raise 'not-found)
(let ([lr (cdr ls)])
(cond
[(null? lr) (raise 'not-found)]
[(null? (cdr lr)) (car ls)]
[else (last_but_one lr)])))))

reverse

(define last_but_one
(lambda (ls)
(if (or (null? ls)
(null? (cdr ls)))
(raise 'not-found)
(cadr (reverse ls)))))

list-ref

(define last_but_one
(lambda (ls)
(let ([len (length ls)])
(if (< len 2)
(raise 'not-found)
(list-ref ls (- len 2))))))

———-更新2015-12-15———–

(define last_but_one
(lambda (ls)
(if (or (null? ls)
(null? (cdr ls)))
(raise 'not-found)
(let loop ([h ls] [t (cddr ls)])
(if (null? t)
(car h)
(loop (cdr h) (cdr t)))))))

—-2015/12/23更新—-

(define last_but_one
(lambda (ls)
(if (or (null? ls)
(null? (cdr ls)))
(raise 'not-found)
(let loop ([h (car ls)]
[s (cadr ls)]
[t (cddr ls)])
(if (null? t)
h
(loop s
(car t)
(cdr t)))))))