public ListNode mergeKLists(ListNode[] lists) {

        ListNode dummy = new ListNode(-1);

        ListNode prev = dummy;

        int k = lists.length;

        PriorityQueue<ListNode> pq = new PriorityQueue<>(new Comparator<ListNode>() {

            @Override

            public int compare(ListNode o1, ListNode o2) {

                return o1.val - o2.val;

            }

        });

        for (int i = 0; i < k; i++) if (lists[i] != null) pq.add(lists[i]);

        while (!pq.isEmpty()) {

            ListNode curNode = pq.poll();

            prev.next = curNode;

            prev = prev.next;

            if (curNode.next != null) pq.add(curNode.next);

        }

        return dummy.next;

    }