var sol = new Solution();

var cases = new (int?[] input, int?[] expected)[]
{
	(new int?[] { 1, 2, 5, 3, 4, null, 6 }, new int?[] { 1, null, 2, null, 3, null, 4, null, 5, null, 6 }),
	// (Array.Empty<int?>(), Array.Empty<int?>()),
	// (new int?[] { 0 }, new int?[] { 0 })
};

foreach (var (input, expected) in cases)
{
	var root = BuildTreeFromLevelOrder(input);
	if (root != null)
		sol.Flatten(root);

	var actual = FlattenToOutputList(root);
	Console.WriteLine($"[{string.Join(", ", input.Select(FormatNullable))}] -> [{string.Join(", ", actual.Select(FormatNullable))}] (expected: [{string.Join(", ", expected.Select(FormatNullable))}])");
}

static TreeNode? BuildTreeFromLevelOrder(int?[] data)
{
	if (data.Length == 0 || data[0] == null)
		return null;

	var root = new TreeNode(data[0]!.Value);
	var queue = new Queue<TreeNode>();
	queue.Enqueue(root);

	var i = 1;
	while (queue.Count > 0 && i < data.Length)
	{
		var node = queue.Dequeue();

		if (i < data.Length && data[i] != null)
		{
			node.left = new TreeNode(data[i]!.Value);
			queue.Enqueue(node.left);
		}
		i++;

		if (i < data.Length && data[i] != null)
		{
			node.right = new TreeNode(data[i]!.Value);
			queue.Enqueue(node.right);
		}
		i++;
	}

	return root;
}

static List<int?> FlattenToOutputList(TreeNode? root)
{
	var result = new List<int?>();
	var node = root;
	while (node != null)
	{
		result.Add(node.val);
		if (node.right != null)
			result.Add(null);
		node = node.right;
	}

	return result;
}

static string FormatNullable(int? value) => value?.ToString() ?? "null";