[LeetCode] Validate Binary Search Tree - Python

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

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.

即是否为二叉查找树。

思路：

一个binary-tree有效，要求root节点的左子树的节点的值都小于root节点的值，root节点的右子树的节点的值都大于root节点的值。所以遍历时只要判断节点的值是否处于该子树的允许范围即可。

代码：

# 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
        """
        return self.helper(root, None, None)

    def helper(self, root, leftbound, rightbound):
        if root is None:
            return True
        if (leftbound and leftbound.val >= root.val or
                rightbound and rightbound.val <= root.val):
            return False

        return (self.helper(root.left, leftbound, root) and
                self.helper(root.right, root, rightbound))