; SIGNATUR
; btree-fast-inorder : btree -> list
; ERKLÄRUNG
; liefert die Liste der Elemente des Baums von links nach rechts
; BEISPIEL
; (btree-fast-inorder empty) == empty
; (btree-fast-inorder (make-branch (make-branch empty 1 empty) 0 empty))
;       == (list 1 0)
; (btree-fast-inorder
;  (make-branch (make-branch empty 5 (make-branch empty 10 empty))
;               11 empty))
;       == (list 5 10 11)
; DEFINITION
(define btree-fast-inorder
  (lambda (t)
    (btree-fast-inorder-1 t empty)))

(define btree-fast-inorder-1
  (lambda (t a)
    (cond
      ((empty? t) a)
      ((branch? t)
       (btree-fast-inorder-1
        (branch-left t)
        (cons (branch-elem t)
              (btree-fast-inorder-1 (branch-right t) a)))))))