【LeetCode OJ】Validate Binary Search Tree

Problem Link:

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

We inorder-traverse the tree, and for each node we check if current_node.val > prev_node.val. The code is as follows.

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

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isValidBST(self, root):
        stack = []
        prev_node = None
        p = root
        while stack or p:
            if p:
                stack.append(p)
                p = p.left
            else:
                p = stack.pop()
                if prev_node:
                    if prev_node.val >= p.val:
                        return False
                prev_node = p
                p = p.right
        return True