合法二叉搜索树

https://leetcode-cn.com/problems/legal-binary-search-tree-lcci/

# 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 __init__(self):
        self.nodes = []

    def isValidBST(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        self.inorder(root)
        return self.nodes == sorted(set(self.nodes))


    def inorder(self, node):
        if not node:
            return
        self.inorder(node.left)
        self.nodes.append(node.val)
        self.inorder(node.right)