import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代碼中的類名、方法名、參數(shù)名已經(jīng)指定，請勿修改，直接返回方法規(guī)定的值即可
     *
     *
     * @param lists ListNode類ArrayList
     * @return ListNode類
     */
    public ListNode mergeKLists (ArrayList<ListNode> lists) {
        // write code here
        Queue<ListNode> priorityQueue = new PriorityQueue<>((v1,
                v2) -> v1.val - v2.val);//創(chuàng)造小根堆
        for (int i = 0; i < lists.size(); i++) {
            if (lists.get(i) != null) {
                priorityQueue.add(lists.get(i));
            }
        }
        ListNode res = new ListNode(-1);
        ListNode head = res;
        while (!priorityQueue.isEmpty()) {
            ListNode tmp = priorityQueue.poll();
            head.next = tmp;
            head = head.next;
            //將tmp鏈表的下一個節(jié)點(diǎn)加入小根堆
            if (tmp.next != null) {
                priorityQueue.add(tmp.next);
            }
        }
        return res.next;
    }
}

最小值問題,多去想象小根堆實(shí)現(xiàn),小根堆的維護(hù)確保了堆頂一定是最小元素