public class Solution
{
    public ListNode? MergeKLists(ListNode[] lists)
    {
        ListNode? head = null;
        ListNode? tail = null;
        var queue = new PriorityQueue<ListNode, int>();
        foreach (var l in lists)
            if (l != null)
                queue.Enqueue(l, l.val);
        while (queue.Count > 0)
        {
            var l = queue.Dequeue();
            if (head == null || tail == null)
            {
                head = l;
                tail = l;
            }
            else
            {
                tail.next = l;
                tail = l;
            }
            if (l.next != null)
                queue.Enqueue(l.next, l.next.val);
        }
        return head;
    }
}