# 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: TreeNode, sm=[], gt=[]) -> bool:

if root is None:

return True

if any([root.val <= x for x in sm]) or any([root.val >= x for x in gt]):

return False

return (root.left is None or self.isValidBST(root.left, sm, gt + [root.val])) and (root.right is None or self.isValidBST(root.right, sm + [root.val], gt))