Merge k sorted linked lists and return it as one sorted list.

public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) {
            return null;
        }
        Queue<ListNode> queue = new PriorityQueue<ListNode>(lists.length, new Comparator<ListNode>() {
            @Override
            public int compare(ListNode o1, ListNode o2) {
                // TODO Auto-generated method stub
                if (o1.val > o2.val) {
                    return 1;
                } else if (o1.val == o2.val) {
                    return 0;
                } else {
                    return -1;
                }
            }           
        });
        for (ListNode listNode : lists) {
            if (listNode != null) {
                queue.add(listNode);
            }
        }
        ListNode head = new ListNode(-1);
        ListNode cur = head;
        while (!queue.isEmpty()) {
            cur.next = queue.poll();
            cur = cur.next;
            if (cur.next != null) {
                queue.add(cur.next);
            }
        }
        return head.next;
    }