#|
 | Asg5Lisp.txt
 | By Vanessa Gerber (hpj766)
 |#

;; removeNILTop is passed a list and removes all NIL at the top level
(defun removeNILTop (L)
   (COND ( (NULL L) NIL )
         ( (NULL (CAR L)) (removeNILTop(CDR L)) ) ;; if head is NIL, remove
         ( T   ( CONS (CAR L) (removeNILTop(CDR L)) ) ) ;; else keep
   )
)

;; removeNILMost is passed a list and removes NIL at any level
(defun removeNILMost (L)
   (COND ( (NULL L) NIL )
         ( (NULL (CAR L)) (removeNILMost(CDR L)) ) ;; if top is NIL, remove
         ( (LISTP (CAR L)) ;; if list
            (CONS (removeNILMost(CAR L)) (removeNILMost(CDR L)))
         )
         ( T   ( CONS (CAR L) (removeNILMost(CDR L)) ) ) ;; else keep
   )
)

;; reverseTop() is passed a list and returns it in reverse
(defun reverseTop (L)
   (COND ( (NULL L) NIL )
      ( ( NULL (CADR L) ) L ) ;; if last element, return that element
      (T    (APPEND (reverseTop(CDR L)) (LIST (CAR L)) ) )
   )
)

;; reverseAll() is passed a list and returns a reversed list all levels
(defun reverseAll (L)
   (COND ( (NULL L) NIL )
      ( ( NULL (CADR L) ) L ) ;; if last element, return that element
      ( (LISTP (CAR L)) ;; if head is list, reverse that too
         (APPEND (reverseAll(CDR L)) (LIST (reverseAll(CAR L))) )
      )
      (T    (APPEND (reverseAll(CDR L)) (LIST (CAR L)) ) )
   )
)

;; palindrome() is passed a list and returns T if list is a palindrome at top lv
(defun palindrome (L)
   ( COND ( (EQUAL (reverseTop L) L) T ) ;; if reverse is equal to list
   (T NIL)) ;; not a palindrome
)

;; [ EXTRA CREDIT ]
;; removeNILAll() removes any resulting NIL except the single outermost
(defun removeNILTop (L)
   (COND ( (NULL L) NIL )
         ( (LISTP (CAR L)) ;; if head is list, recurse
            (APPEND (removeNILAll (CAR L)) (removeNILAll (CDR L)))
         )
         ( (NULL (CAR L)) (removeNILAll(CDR L)) ) ;; if head is NIL, remove
         ( T   ( CONS (CAR L) (removeNILAll(CDR L)) ) ) ;; else keep
   )
)