Leetcode解题-Validate Binary Search Tree

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描述

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.

分析

检查一个二叉搜索树是不是合法，递归去做，向下传递一个允许的取值范围。时间O(n)，空间平均O(logn)。

代码

Python

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class Solution(object):
    def isValidBSTR(self, root, low, high):
        if not root:
            return True
        if root.val >= high or root.val <= low:
            return False
        if not self.isValidBSTR(root.left, low, root.val):
            return False
        if not self.isValidBSTR(root.right, root.val, high):
            return False
        return True

    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        return self.isValidBSTR(root, float('-inf'), float('inf'))

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