# [Solved] Given the root of a binary tree, determine if it is a valid binary search tree (BST).

## Question

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

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:

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

Example 2:

```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`

## Python Solution

```class Solution:
def isValidBST(self, root: TreeNode,l=float('-inf'),r=float('inf')) -> bool:
if not root:
return True
if root.val<=l or root.val>=r:
return False
return self.isValidBST(root.left,l,min(r,root.val)) and self.isValidBST(root.right,max(l,root.val),r)```