2.64.scm

(define (make-tree entry left-branch right-branch)
  (list entry left-branch right-branch))

(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))))))))

; a)

(list->tree (list 1 3 5 7 9 11))

;    5
;  /   \
; 1     9
;  \   / \
;   3 7   11

; b) O(n)