合并K个排序链表

题目来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/merge-k-sorted-lists

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

Java代码：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        ListNode tmp = null;
        for(int i = 0;i<lists.length;i++){
            ListNode cur = lists[i]; 
            tmp = mergeTwo(cur,tmp);
        }
        return tmp;
    }

    private ListNode mergeTwo(ListNode l1,ListNode l2){
        if(l1 == null)
            return l2;
        if(l2 == null)
            return l1;
        ListNode ans = new ListNode(0);
        ListNode cur = new ListNode(0);
        ans.next = cur;
        while(l1 != null || l2 != null){
            if(l1 == null){
                cur.next = l2;
                break;
            }
            if(l2 == null){
                cur.next = l1;
                break;
            }
            if(l1.val > l2.val){
                cur.next = l2;
                l2 = l2.next;
            }else{
                cur.next = l1;
                l1 = l1.next;
            }
            cur = cur.next;
        }
        return ans.next.next;
    }
}