(define make-empty-tree
  (lambda () '()))

(define make-nonempty-tree
  (lambda (root left-subtree right-subtree)
    (list root left-subtree right-subtree)))

(define example-tree
  (make-nonempty-tree
   191
   (make-nonempty-tree 157
                       (make-empty-tree)
                       (make-empty-tree))
   (make-nonempty-tree 202
                       (make-nonempty-tree 200
                                           (make-empty-tree)
                                           (make-empty-tree))
                       (make-empty-tree))))


(define empty-tree? null?)

(define root car)

(define left-subtree cadr)

(define right-subtree caddr)

(define in?
  (lambda (value tree)
    (cond
     ((empty-tree? tree) #f)
     ((= value (root tree)) #t)
     ((< value (root tree)) (in? value (left-subtree tree)))
     (else ; the value must be greater than the root
      (in? value (right-subtree tree))))))

(define tree-maximum
  (lambda (tree)
    ; tree must be non-empty
    (cond ((empty-tree? tree)
           (error "this doesn't make sense -- there is no element, so how could one be largest?"))
          ((empty-tree? (right-subtree tree))
           (root tree))
          (else
           (tree-maximum (right-subtree tree))))))

(define inorder
  (lambda (tree)
    (if (empty-tree? tree)
        '()
        (append (inorder (left-subtree tree))
                (cons (root tree) 
                      (inorder (right-subtree tree)))))))