# [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)``` ##### Abhishek Sharma

Started my Data Science journey in my 2nd year of college and since then continuously into it because of the magical powers of ML and continuously doing projects in almost every domain of AI like ML, DL, CV, NLP.

Articles: 521