Convert BST to Greater Tree

1def convertBST(root: Optional[TreeNode]) -> Optional[TreeNode]:
2    total = 0
3    def dfs(node: Optional[TreeNode]) -> None:
4        nonlocal total
5        if not node:
6            return
7        dfs(node.right)
8        total += node.val
9        node.val = total
10        dfs(node.left)
11    dfs(root)
12    return root