098 Validate Binary Search Tree.py

'''
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.
'''

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


class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        stack = []
        curr = root
        prev = None
        while curr or stack:
            while curr:
                stack.append(curr)
                curr = curr.left

            if stack:
                curr = stack.pop()
                if prev and curr.val <= prev.val:
                    return False
                prev = curr
                curr = curr.right
        return True


if __name__ == "__main__":
    None