Validate Binary Search Tree – Python 3 (Week 18 – 04)

"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""

class Solution:
    """
    @param root: The root of binary tree.
    @return: True if the binary tree is BST, or false
    """
    def isValidBST(self, root):
        # write your code here
        if root is None:
            return True
        
        stack = []
        
        while root:
            stack.append(root)
            root = root.left
        
        last_node = stack[-1]
        while stack:
            node = stack.pop()
            if node.right:
                node = node.right
                while node:
                    stack.append(node)
                    node = node.left
                    
            if stack:
                if stack[-1].val <= last_node.val:
                    return False
                last_node = stack[-1]
                
        return True