合并 k\ k k 个已排序的链表并将其作为一个已排序的链表返回。

基于合并两个链表的基础上就行，对于K个>2个链表，则依次取出一个将其与已经合并好的就行，没什么难度

import java.util.ArrayList;
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ArrayList<ListNode> lists) {
    ListNode newList = null;
        if (lists.size() == 1) {
            return lists.get(0);
        } else if (lists.size() >= 2) {
            newList = merge2Lists(lists.get(0), lists.get(1));
            for (int i = 2; i < lists.size(); i++) {
                newList = merge2Lists(newList, lists.get(i));
            }
        }

        return newList;
    }

    public  ListNode merge2Lists(ListNode list1, ListNode list2) {
        ListNode newlist = new ListNode(0);
        ListNode n1 = newlist;
        ListNode l1 = list1;
        ListNode l2 = list2;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                n1.next = l1;
                n1 = n1.next;
                l1 = l1.next;
                n1.next = null;
            } else {
                n1.next = l2;
                n1 = n1.next;
                l2 = l2.next;
                n1.next = null;
            }

        }
        while (l1 != null) {
            n1.next = l1;
            l1 = l1.next;
            n1 = n1.next;
        }
        while (l2 != null) {
            n1.next = l2;
            l2 = l2.next;
            n1 = n1.next;
        }
        return newlist.next;
    }
}