All Elements in Two Binary Search Trees LeetCode solution in Python

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# Definition for a binary tree node.
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# class TreeNode:
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#     def __init__(self, val=0, left=None, right=None):
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#         self.val = val
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#         self.left = left
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#         self.right = right
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class Solution:
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    def getAllElements(self, root1: TreeNode, root2: TreeNode) -> List[int]:
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        l1, l2 = [], []
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        def preorder(root, l):
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            if root is None:
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                return
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            preorder(root.left, l)
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            l.append(root.val)
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            preorder(root.right, l)
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        preorder(root1, l1)
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        preorder(root2, l2)
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        return sorted(l1 + l2)