优先队列

    public ListNode mergeKLists(ListNode[] lists) {
        if(lists==null||lists.length==0)
            return null;
        PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(lists.length,new Comparator<ListNode>(){
            @Override
            public int compare(ListNode o1,ListNode o2){
                if(o1.val<o2.val)
                    return -1;
                else if(o1.val==o2.val)
                    return 0;
                else
                    return 1;
            }
        });
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        for(ListNode node:lists)
            if(node!=null)
                queue.add(node);
        while(!queue.isEmpty()){
            tail.next = queue.poll();
            tail = tail.next;
            if(tail.next!=null)
                queue.add(tail.next);
        }
        return dummy.next;
    }