Merge k sorted list

1. mistake made: how to write a class that can be fed to PriorityQueue

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class QueueNode implements Comparable<QueueNode> {
    public ListNode ln;
    public QueueNode(ListNode n){
        ln = n;
    }
    public int compareTo(QueueNode other){
        return this.ln.val - other.ln.val;
    }
}
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        ListNode head = null, prev = null;
        if (lists.length == 0){
            return head;
        }
        PriorityQueue<QueueNode> heap = new PriorityQueue<>();
        for (ListNode ln : lists){
            if (ln!=null )
                heap.offer(new QueueNode(ln));
        }
        while (!heap.isEmpty()){
            QueueNode qn = heap.poll();
            ListNode crt = qn.ln;
            if (qn.ln.next != null){
                qn.ln = qn.ln.next;
                heap.offer(qn);
            }
            crt.next = null;
            if (head == null){
                head = crt;
            }
            if (prev!=null){
                prev.next = crt;
            }
            prev = crt;
        }
        return head;
    }
}