Exercise 2.64: The following procedure
list->tree converts an ordered list to a balanced binary tree.  The
helper procedure partial-tree takes as arguments an integer 
  n
 and
list of at least 
  n
 elements and constructs a balanced tree containing the
first 
  n
 elements of the list.  The result returned by partial-tree
is a pair (formed with cons) whose car is the constructed tree
and whose cdr is the list of elements not included in the tree.


(define (list->tree elements)
  (car (partial-tree 
        elements (length elements))))

(define (partial-tree elts n)
  (if (= n 0)
      (cons '() elts)
      (let ((left-size 
             (quotient (- n 1) 2)))
        (let ((left-result 
               (partial-tree 
                elts left-size)))
          (let ((left-tree 
                 (car left-result))
                (non-left-elts 
                 (cdr left-result))
                (right-size 
                 (- n (+ left-size 1))))
            (let ((this-entry 
                   (car non-left-elts))
                  (right-result 
                   (partial-tree 
                    (cdr non-left-elts)
                    right-size)))
              (let ((right-tree 
                     (car right-result))
                    (remaining-elts 
                     (cdr right-result)))
                (cons (make-tree this-entry 
                                 left-tree 
                                 right-tree)
                      remaining-elts))))))))


 Write a short paragraph explaining as clearly as you can how
partial-tree works.  Draw the tree produced by list->tree for
the list (1 3 5 7 9 11).

 What is the order of growth in the number of steps required by
list->tree to convert a list of 
  n
 elements?