; SIGNATUR
; btree-preorder : btree -> list
; ERKLÄRUNG
; liefert die Liste der Elemente des Baums in Preorder-Reihenfolge
; BEISPIEL
; (btree-preorder empty) == empty
; (btree-preorder (make-branch (make-branch empty 1 empty) 0 empty))
;       == (list 0 1)
; (btree-preorder
;  (make-branch (make-branch empty 5 (make-branch empty 10 empty))
;                             11 empty))
;       == (list 11 5 10)
; DEFINITION
(define btree-preorder
  (lambda (t)
    (cond
      ((empty? t) empty)
      ((branch? t)
       (cons (branch-elem t)
             (append (btree-preorder (branch-left t))
                    (btree-preorder (branch-right t))))))))


; SIGNATUR
; btree-postorder : btree -> list
; ERKLÄRUNG
; liefert die Liste der Elemente des Baums in Postorder-Reihenfolge
; BEISPIEL
; (btree-postorder empty) == empty
; (btree-postorder (make-branch (make-branch empty 1 empty) 0 empty))
;       == (list 1 0)
; (btree-postorder
;  (make-branch (make-branch empty 5 (make-branch empty 10 empty))
;                             11 empty))
;       == (list 5 10 11)
; DEFINITION
(define btree-postorder
  (lambda (t)
    (cond
      ((empty? t) empty)
      ((branch? t)
       (append (btree-postorder (branch-left t))
               (btree-postorder (branch-right t))
               (list (branch-elem t)))))))