说明：  二叉排序树或者是一棵空树，或者是具有下列性质的二叉树： （1）若左子树不空，则左子树上所有结点的值均小于或等于它的根结点的值； （2）若右子树不空，则右子树上所有结点的值均大于或等于它的根结点的值； （3）左、右子树也分别为二叉排序树；
(A two - fork sort tree or an empty tree, or a two - fork tree with the following properties: (1) if the left subtree is not empty, the value of all the nodes on the left subtree is less than or equal to the value of its root node. (2) if the right subtree is not empty, the value of all the nodes on the right subtree is greater than or equal to the value of its root node. (3) the left and right subtrees are also two forked trees, respectively.)