[leetcode] Validate Binary Search Tree （检验是否为二叉查找树） Python

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.

leetcode已经给出了二叉查找树的定义了。

class Solution:
    # @param root, a tree node
    # @return a boolean
    def isValidBST(self, root):
        if root is None:
            return True
        return self.helper(root,-99999,999999)

    # left ~ right 是node的取值范围
    def helper(self,root,left,right):

        if root is None:
            return True
        if root.val <= left or root.val >=right:
            #超出范围
            return False
        return self.helper(root.left,left,root.val) and self.helper(root.right,root.val,right)

找到画图工具后我再来更新部分的图