LeetCode-98. Validate Binary Search Tree

https://leetcode.com/problems/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.

Solution

方法一：直接中序遍历二叉搜索树，判断得到的数组是不是有序的。

方法二：

# 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 = []
        pre = None
        while len(stack) != 0 or root != None:
            while root != None:
                stack.append(root)
                root = root.left
            root = stack.pop()
            if pre != None and pre.val >= root.val:
                return False
            pre = root
            root = root.right
        return True