[LeetCode] Merge K Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

时间复杂度 O(nlogk)

思路：

这道题其实有三种做法，第一种是priorityQueue，第二种是Divided & Conquer， 第三种是两两并归。每一种都很重要，需要多多联系。

第一种， priorityQueue

/**
 * 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;
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;
        PriorityQueue<ListNode> q = new PriorityQueue<>(lists.length,
        new Comparator<ListNode>(){
            public int compare(ListNode l1, ListNode l2) {
                return l1.val - l2.val;
            }
        });
        
        for (ListNode list : lists) {
            if (list != null) {
                q.offer(list);
            }    
        }
        
        while (!q.isEmpty()) {
            ListNode tmp = q.poll();
            head.next = tmp;
            if (tmp.next != null) {
                q.offer(tmp.next);
            }
            head = head.next;
        }
        return dummy.next;
    }
}