2.64.scm

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

;;   Takes an integer n arguments 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 (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))))))))