这道题一定要理解dfs是递归到了叶子节点然后回溯，否则就做不出来

Py:

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# 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 splitBST(self, root, V):
        
        if not root: return [None, None]
        
        if root.val > V:
            left, right = self.splitBST(root.left, V)
            root.left = right
            return [left, root]
        else :
            left, right = self.splitBST(root.right, V)
            root.right = left
            return [root, right]