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# 98. Validate Binary Search Tree

## Medium

***

Given the `root` of a binary tree, *determine if it is a valid binary search tree (BST)*.

A **valid 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:**

![](https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg)

```
Input: root = [2,1,3]
Output: true
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg)

```
Input: root = [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.
```

 

**Constraints:**

* The number of nodes in the tree is in the range `[1, 104]`.
* `-231 <= Node.val <= 231 - 1`