1+ # Given the root of a binary tree, determine if it is a valid binary search tree (BST).
2+ 3+ # A valid BST is defined as follows:
4+ 5+ # The left subtree of a node contains only nodes with keys less than the node's key.
6+ # The right subtree of a node contains only nodes with keys greater than the node's key.
7+ # Both the left and right subtrees must also be binary search trees.
8+ 9+ 10+ # Example 1:
11+ # Input: root = [2,1,3]
12+ # Output: true
13+ 14+ # Example 2:
15+ # Input: root = [5,1,4,null,null,3,6]
16+ # Output: false
17+ # Explanation: The root node's value is 5 but its right child's value is 4.
18+ 19+ 20+ # Constraints:
21+ 22+ # The number of nodes in the tree is in the range [1, 104].
23+ # -231 <= Node.val <= 231 - 1
24+ 25+ 26+ from typing import Optional
27+ # Definition for a binary tree node.
28+ class TreeNode :
29+ def __init__ (self , val = 0 , left = None , right = None ):
30+ self .val = val
31+ self .left = left
32+ self .right = right
33+ class Solution :
34+ def isValidBST (self , root : Optional [TreeNode ]) -> bool :
35+ return self .checkValidity (root , float ('-inf' ), float ('inf' ))
36+ 37+ def checkValidity (self , node , min_val , max_val ):
38+ if not node :
39+ return True
40+ if node .val < min_val or node .val > max_val :
41+ return False
42+ 43+ return self .checkValidity (node .left , min_val , node .val - 1 ) and \
44+ self .checkValidity (node .right , node .val + 1 , max_val )
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