validate_binary_search_tree.py

"""
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

"""

# 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]) -> bool:
        def is_valid(node, minn, maxx):
            if not node:
                return True
            
            if node.val <= minn or node.val >= maxx:
                return False
            
            return is_valid(node.left, minn, node.val) and is_valid(node.right, node.val, maxx)
        return is_valid(root, float('-inf'), float('inf'))