Created
October 21, 2014 00:00
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;;; jffmcc's solution | |
(define (Last2Equal? L) | |
(cond ((> (length L) 2) (Last2Equal? (cdr L))) | |
((= (length L) 2) (equal? (car L) (cadr L))) | |
(else #f))) | |
;;; Alternative Solutions | |
;;; Compute length only once | |
(define (drop n l) | |
(if (zero? n) | |
l | |
(drop (- n 1) (cdr l)))) | |
(define (last-2-equal? l) | |
(define len (length l)) | |
;; only compute length once, use it twice | |
(if (< len 2) | |
#f | |
(let ((last-2 (drop (- len 2) l))) | |
(equal? (car last-2) (cadr last-2))))) | |
;;; No length computation at all, just list ops | |
(define (last-2-equal? l) | |
(cond ((null? l) #f) | |
((null? (cdr l)) #f) ; length 1 | |
((null? (cddr l)) ; length 2 | |
(equal? (car l) (cadr l))) | |
(else ; length > 2 | |
(last-2-equal? (cdr l))))) | |
(last-2-equal? (list 1 2 3 4 4)) | |
(last-2-equal? (list 1 2 3 4 5)) | |
(last-2-equal? (list 2 2)) | |
(last-2-equal? (list 1)) | |
(last-2-equal? (list)) |
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