[LeetCode] 98. Validate Binary Search Tree

题：https://leetcode.com/problems/validate-binary-search-tree/description/](https://leetc
ode.com/problems/validate-binary-search-tree/description/)

题目

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:

Input:
2
/ \
1 3
Output: true
Example 2:

5

/ \
1 4
/ \
3 6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node’s value
is 5 but its right child’s value is 4.

思路

这里用到了一个性质，对于 Binary Search Tree ，它的中序遍历是严格单调递增。

另外， 可能使用用动态规划的想法，只要 左树的max < 当前节点 < 右树的min，具体的实现还要进一步思考。

code

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

class Solution:
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        tstack = []
        node = root
        tmp = None
        while node != None or len(tstack) != 0:
            while node != None:
                tstack.append(node)
                node = node.left
            node = tstack.pop(-1)
            print(node.val)
            if tmp != None:
                if not tmp<node.val:
                    return False
            tmp = node.val
            node = node.right
        return True