ValidateBST.py

#   Created by Elshad Karimov 
#   Copyright © AppMillers. All rights reserved.

# Validate BST

class TreeNode:
     def __init__(self, value):
         self.val = value
         self.left = None
         self.right = None

def helper(node, minValue = float('-inf'), maxValue = float('inf')):
    if not node:
        return True
    val = node.val
    if val <= minValue or val >= maxValue:
        return False
    
    if not helper(node.right, val, maxValue):
        return False
    
    if not helper(node.left, minValue, val):
        return False
    
    return True

def isValidBST(root):
    return helper(root)

root1 = TreeNode(2)
root1.left = TreeNode(1)
root1.right = TreeNode(4)

print(isValidBST(root1))

root2 = TreeNode(4)
root2.left = TreeNode(1)
root2.right = TreeNode(3)

print(isValidBST(root2))