LeetCode Convert BST to Greater Tree: how to do without GenServer?

Questions & Help Questions
5

I came up with a similar solution, and it passed the tests:

defmodule Solution do
  @spec convert_bst(root :: TreeNode.t | nil) :: TreeNode.t | nil  
  def convert_bst(root) do
    {converted, _} = do_convert_bst(root, 0)
    converted
  end
    
  defp do_convert_bst(nil, _carry), do: {nil, 0}
  defp do_convert_bst(node, carry) do
    {right, sum_r} = do_convert_bst(node.right, carry)
    carry = carry + node.val + sum_r
    {left, sum_l} = do_convert_bst(node.left, carry)
    {%TreeNode{left: left, right: right, val: carry}, node.val + sum_l + sum_r}
  end
end

carry is the sum of the values of all the nodes that have already been visited that have values greater than the next node to visit.