function sumBinaryTree(tree: (number | null)[], index: number = 0): number {

  if (tree.length === 0) return 0

  return tree[0] === null
    ? 0 + sumBinaryTree(tree.slice(1))
    : tree[0] + sumBinaryTree(tree.slice(1))
}

// No modificar: necesario para evaluar el resultado.
export { sumBinaryTree };