last2.scm

;;; 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))