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Section

# Definition for a binary tree node.

defmodule TreeNode do
  @type t :: %__MODULE__{
          val: integer,
          left: TreeNode.t() | nil,
          right: TreeNode.t() | nil
        }
  defstruct val: 0, left: nil, right: nil
end

defmodule Solution do
  @spec is_balanced(root :: TreeNode.t() | nil) :: boolean
  def is_balanced(root) do
    traverse(root)
  end

  def traverse(node) when node.left == nil and node.right == nil do
    1
  end

  def traverse(node) when node.left == nil or node.right == nil do
    1
  end

  def traverse(node) do
    abs(traverse(node.left) + traverse(node.right))
  end

  # def to_native_list(nil), do: []

  # def to_native_list(head) do
  #   [head.val | to_native_list(head.next)]
  # end
end

node = %TreeNode{
  val: 1,
  left: %TreeNode{
    val: 2,
    right: %TreeNode{val: 3, left: nil, right: nil},
    left: %TreeNode{
      val: 3,
      right: %TreeNode{val: 4, left: nil, right: nil},
      left: %TreeNode{val: 4, left: nil, right: nil}
    }
  },
  right: %TreeNode{left: nil, right: nil, val: 2}
}

Solution.is_balanced(node)