SICP 2.35

;; Flatten tree then count
(define (count-leaves0 t)
  (accumulate (lambda (x total) (+ total 1)) 0 
              (map (lambda (children) children) 
                   (enumerate-tree t))))


(define (count-leaves1 t)
  (accumulate + 0 
              (map (lambda (children)
                     (if (pair? children)
                         (count-leaves1 children)
                         1))
                   t)))