Validate Binary Search Tree

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

Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key. The right subtree of a node contains only nodes with keys greater than the node's key. Both the left and right subtrees must also be binary search trees. Example 1: 2 / \ 1 3 Binary tree [2,1,3], return true. Example 2: 1 / \ 2 3 Binary tree [1,2,3], return false.

URL: https://leetcode.com/problems/validate-binary-search-tree/

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
import sys
class Solution:
    def __init__(self):
        self.lastPrinted = -sys.maxsize-1
    # @param {TreeNode} root
    # @return {boolean}
    def isValidBST(self, root):
        if root == None:
            return True

        if self.isValidBST(root.left) == False:
            return False

        data = root.val
        if data <= self.lastPrinted:
            return False

        self.lastPrinted = data

        if self.isValidBST(root.right) == False:
            return False

        return True