23. 合并K个排序链表（堆排序）.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) {
            return null;
        }
        // 创建一个堆，并设置元素的排序方式
        PriorityQueue<ListNode> queue = new PriorityQueue(new Comparator<ListNode>() {
            public int compare(ListNode l1, ListNode l2) {
                return l1.val - l2.val;
            }
        });
        // 遍历链表数组，然后将每个链表的每个节点都放入堆中
        for (int i = 0; i < lists.length; i++) {
            while (lists[i] != null) {
                queue.add(lists[i]);
                lists[i] = lists[i].next;
            }
        }
        ListNode dummy = new ListNode(-1);
        ListNode head = dummy;
        // 从堆中不断取出元素，并将取出的元素串联起来
        while (!queue.isEmpty()) {
            dummy.next = queue.poll();
            dummy = dummy.next;
        }
        dummy.next = null;
        return head.next;
    }
}