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合法二叉搜索树
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https://leetcode-cn.com/problems/legal-binary-search-tree-lcci/

.. code:: python

    # 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)