/**
<p>给你一个链表数组，每个链表都已经按升序排列。</p>

<p>请你将所有链表合并到一个升序链表中，返回合并后的链表。</p>

<p>&nbsp;</p>

<p><strong>示例 1：</strong></p>

<pre><strong>输入：</strong>lists = [[1,4,5],[1,3,4],[2,6]]
<strong>输出：</strong>[1,1,2,3,4,4,5,6]
<strong>解释：</strong>链表数组如下：
[
  1->4->5,
  1->3->4,
  2->6
]
将它们合并到一个有序链表中得到。
1->1->2->3->4->4->5->6
</pre>

<p><strong>示例 2：</strong></p>

<pre><strong>输入：</strong>lists = []
<strong>输出：</strong>[]
</pre>

<p><strong>示例 3：</strong></p>

<pre><strong>输入：</strong>lists = [[]]
<strong>输出：</strong>[]
</pre>

<p>&nbsp;</p>

<p><strong>提示：</strong></p>

<ul>
  <li><code>k == lists.length</code></li>
  <li><code>0 <= k <= 10^4</code></li>
  <li><code>0 <= lists[i].length <= 500</code></li>
  <li><code>-10^4 <= lists[i][j] <= 10^4</code></li>
  <li><code>lists[i]</code> 按 <strong>升序</strong> 排列</li>
  <li><code>lists[i].length</code> 的总和不超过 <code>10^4</code></li>
</ul>
<div><div>Related Topics</div><div><li>链表</li><li>分治</li><li>堆（优先队列）</li><li>归并排序</li></div></div><br><div><li>???? 1874</li><li>???? 0</li></div>
*/

//leetcode submit region begin(Prohibit modification and deletion)

import javax.xml.transform.Templates;
import java.util.PriorityQueue;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
    if(lists.length==0){
      return null;
    }

    PriorityQueue<ListNode> pq = new PriorityQueue<ListNode>(
        lists.length,(a,b)->(a.val - b.val)
    );

    ListNode dummy = new ListNode(-1);
    ListNode p = dummy;

    //通过优先级队列， 小根堆， 排序头节点
    for(ListNode head : lists){
      if(head != null){
        pq.add(head);
      }
    }

    while(!pq.isEmpty()){
      ListNode temp = pq.poll();
      p.next = temp;

      //每次排序完一个节点，然后放入这个节点的下一个节点
      if(temp.next!=null){
        pq.add(temp.next);
      }
      p = p.next;

    }
    return dummy.next;
    }
}
//leetcode submit region end(Prohibit modification and deletion)