gistfile1.lisp

;; slow because appending to a list means that
;; we have to traverse it over and over again
(defun my-rev-1 (lst)
  (if (null lst) 
    '() 
    (append (my-rev-1 (cdr lst)) (list (car lst)))))

;; better because we use an empty list that we can
;; cons onto the front of (which is fast)
;; "labels" is a way to make a private recursive
;; function
(defun my-rev-2 (lst)
  (labels ((rec (list new-list)
              (if (null list)
                new-list
                (rec (cdr list) (cons (car list) new-list)))))
    (rec lst '())))