## 题目

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

Binary tree [2,1,3], return true.

Example 2:

Binary tree [1,2,3], return false.

## 要求

1. 一个节点的左子树的所有节点都小于（不能等于）该节点的值；
2. 一个节点的右子树的所有节点都大于（不能等于）该节点的值；
3. 左右子树也必须是二叉搜索树。