问题链接

https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/625/

解题思路

• 二叉树中序遍历
• 注意点：初始变量不能写在递归函数内，不然每次都会被初始化为None

错误答案

class Solution(object):
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        preNode = None
        if root:

            if not self.isValidBST(root.left):
                return False

            if preNode and preNode.val >= root.val:
                return False

            preNode = root

            if not self.isValidBST(root.right):
                return False

        return True

正确答案

class Solution(object):
    preNode = None
    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """

        if root:

            if not self.isValidBST(root.left):
                return False

            if self.preNode and self.preNode.val >= root.val:
                return False

            self.preNode = root

            if not self.isValidBST(root.right):
                return False

        return True