Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST). Tags: Tree

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# 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) -> bool:
        return self.helper(root, float('-inf'), float('inf'))
    
    def helper(self, root, lower, upper):
        if root is None:
            return True
        elif root.val >= upper:
            return False
        elif root.val <= lower:
            return False
        else:
            return self.helper(root.left, lower, root.val) and self.helper(root.right, root.val, upper)