[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).

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

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)