# [98. Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)


## 题目

Given a binary tree, determine if it is a valid binary search tree (BST).

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.

**xample 1:**

        2
       / \
      1   3
    
    Input: [2,1,3]
    Output: true

**Example 2:**

        5
       / \
      1   4
         / \
        3   6
    
    Input: [5,1,4,null,null,3,6]
    Output: false
    Explanation: The root node's value is 5 but its right child's value is 4.

## 题目大意

给定一个二叉树，判断其是否是一个有效的二叉搜索树。假设一个二叉搜索树具有如下特征：

- 节点的左子树只包含小于当前节点的数。
- 节点的右子树只包含大于当前节点的数。
- 所有左子树和右子树自身必须也是二叉搜索树。


## 解题思路

- 判断一个树是否是 BST，按照定义递归判断即可