public ListNode BuildList(int[] input,int index=0)
{
    return index+1 < input.Length ? new ListNode(input[index], BuildList(input, ++index)) : new ListNode(input[index]);
}

public int[] Output(ListNode n)
{
    var res = new List<int>();
    var node = n;
    while (node != null)
    {
        res.Add(node.val);
        node = node.next;
    }

    return res.ToArray();
}