【23. 合并K个排序链表】【困难】【优先队列/堆排序】

合并 k 个排序链表，返回合并后的排序链表。请分析和描述算法的复杂度。

示例:

输入:
[
1->4->5,
1->3->4,
2->6]
输出: 1->1->2->3->4->4->5->6

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        public class Solution {
    public ListNode mergeKLists(List<ListNode> lists) {
        Queue<ListNode> heap = new PriorityQueue(new Comparator<ListNode>(){
            @Override public int compare(ListNode l1, ListNode l2) { 
                return l1.val - l2.val; 
            }
        });
        ListNode head = new ListNode(0), tail = head;
        for (ListNode node : lists) if (node != null) heap.offer(node);
        while (!heap.isEmpty()) {
            tail.next = heap.poll();
            tail = tail.next;
            if (tail.next != null) heap.offer(tail.next);
        }
        return head.next;
    }
}
    }
}