二叉树删除 lisp

;;; From ANSI Common Lisp

; If you have questions or comments about this code, or you want
; something I didn't include, send mail to lispcode@paulgraham.com.

; This code is copyright 1995 by Paul Graham, but anyone who wants
; to use it is free to do so.

(defun bst-remove (obj bst <)
(if (null bst)
nil
(let ((elt (node-elt bst)))
(if (eql obj elt)
(percolate bst)
(if (funcall < obj elt)
(make-node
:elt elt
:l (bst-remove obj (node-l bst) <)
:r (node-r bst))
(make-node
:elt elt
:r (bst-remove obj (node-r bst) <)
:l (node-l bst)))))))

(defun percolate (bst)
(let ((l (node-l bst)) (r (node-r bst)))
(cond ((null l) r)
((null r) l)
(t (if (zerop (random 2))
(make-node :elt (node-elt (bst-max l))
:r r
:l (bst-remove-max l))
(make-node :elt (node-elt (bst-min r))
:r (bst-remove-min r)
:l l))))))

(defun bst-remove-min (bst)
(if (null (node-l bst))
(node-r bst)
(make-node :elt (node-elt bst)
:l (bst-remove-min (node-l bst))
:r (node-r bst))))

(defun bst-remove-max (bst)
(if (null (node-r bst))
(node-l bst)
(make-node :elt (node-elt bst)
:l (node-l bst)
:r (bst-remove-max (node-r bst)))))