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Validate Binary Search Tree

Question​

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

Example 1
Input: [2, 1, 3] 
Output: True

Solution​

all//Validate Binary Search Tree.py


# Definition for a binary tree node.
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def isValidBST(self, root):
        return self.helper(root, float('-inf'), float('inf'))

    def helper(self, node, lower, upper):
        if not node:
            return True
        if node.val <= lower or node.val >= upper:
            return False
        return self.helper(node.right, node.val, upper) and self.helper(node.left, lower, node.val)