function sumBinaryTree(tree: (number | null)[], index: number = 0): number {
  if (index >= tree.length || tree[index] === null) return 0

  const node = tree[index]

  // Node + left tree + right tree
  return node
    + sumBinaryTree(tree, 2 * index + 1) // Left
    + sumBinaryTree(tree, 2 * index + 2) // Right
}

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