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)