python
# 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 helper(node, min_val=float('-inf'), max_val=float('inf')):
if not node:
return True
if not (min_val < node.val < max_val):
return False
return helper(node.left, min_val, node.val) and helper(node.right, node.val, max_val)
return helper(root)