Convert BST to Greater Tree 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 convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
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        ans = []
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        def inorder(node):
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            if not node:
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                return node
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            inorder(node.left)
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            ans.append(node.val)
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            inorder(node.right)
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        def dfs(node):
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            if not node:
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                return None
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            idx = ans.index(node.val)
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            node.val = node.val + sum(ans[idx + 1 :])
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            dfs(node.left)
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            dfs(node.right)
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        inorder(root)
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        dfs(root)
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        return root