L-99 http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html

L-99.lisp

;; -*- mode: lisp-interaction -*-
;; http://www.ic.unicamp.br/~meidanis/courses/mc336/2006s2/funcional/L-99_Ninety-Nine_Lisp_Problems.html

;; P01
(defun my-last (lst)
  (if (null lst)
    nil
	(if (null (cdr lst))
		lst
	  (my-last (cdr lst)))))
;; (my-last '(a b c d)) ;=> (d)


;; P02
(defun my-but-last (lst)
  (if (null lst)
	  nil
	(if (<= (length lst) 2)
		lst
	  (my-but-last (cdr lst)))))
;; (my-but-last '(a b c d)) ;=> (c d)


;; P03
(defun my-element-at (lst idx)
  (if (or (>= idx (length lst)) (> 0 idx))
	  nil
	(if (= idx 1)
		(car lst)
	  (my-element-at (cdr lst) (decf idx)))))
;; (my-element-at '(a b c d e) 3) ;=> c


;; P04
(defun my-length(lst)
  (if (null lst)
	  0
	(+ (my-length (cdr lst)) 1)))
;; (my-length '(a b c)) ;=> 3


;; P05
(defun my-reverse (lst &optional (rev-lst '()))
  (if (null lst)
	  rev-lst
	(my-reverse (cdr lst) (cons (car lst) rev-lst))))
;; (my-reverse '(a b c)) ;=> (c b a)


;; P06
(defun my-palindrome-p (lst)
  (equal lst (reverse lst)))
;; (my-palindrome-p '(a b c b a)) ;=> t
;; (my-palindrome-p '(a b c b)) ;=> nil