## DEV Community

Abhishek Chaudhary

Posted on

# Validate Binary Search Tree

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:

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`

SOLUTION:

``````# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
def isValidBST(self, root: Optional[TreeNode], minVal = float('-inf'), maxVal = float('inf')) -> bool:
if root:
if root.val <= minVal or root.val >= maxVal:
return False
if not self.isValidBST(root.left, minVal = minVal, maxVal = min(maxVal, root.val)) or not self.isValidBST(root.right, minVal = max(minVal, root.val), maxVal = maxVal):
return False
return True
``````