098.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.

# 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
        """
        prev = -float("inf")
        stack = [(1, root)]
        while stack:
            p = stack.pop()
            if not p[1]:
                continue
            if p[0] == 0:
                if p[1].val <= prev:
                    return False
                prev = p[1].val
            else:
                stack.append((1, p[1].right))
                stack.append((0, p[1]))
                stack.append((1, p[1].left))
        return True