[LeetCode]题解（python）：098 Validate Binary Search Tree

题目来源

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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.

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题意分析

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Input:二叉树

Output:boolean值

Conditions:检验一个二叉树是不是有效的二叉搜索树

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题目思路

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注意到，每层的最大最小约束都是不一样的，所以采用dfs的思想，不断更新最大最小值。在python中，数字可以取很大，要设大一点。

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AC代码（Python）

# 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 validBST(self, root, min, max):
        if root == None:
            return True
        if root.val <= min or root.val >= max:
            return False
        return self.validBST(root.left, min, root.val) and self.validBST(root.right, root.val, max)
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        return self.validBST(root, -21474836480, 21474836470)