NCNU LISP HW9

: finding the n-th leaf of a tree

(define (enumerate-tree tree)
  (cond ((null? tree) nil)
        ((not (pair? tree)) (list tree))
        (else (append (enumerate-tree (car tree))
                      (enumerate-tree (cdr tree))))))
(define nil '())

(define x (list 1 (list 2 (list 3 4)) 5))

(define (tree-ref L n)
(do-tree-ref L n 1)
)
(define (do-tree-ref L n flag)
(cond ((= flag 1) (do-tree-ref (enumerate-tree L) n 0))
     ((= n 0) (car L))
     (else (do-tree-ref (cdr L) (- n 1) 0))
)
)