leetcode

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-2→…
You must do this in-place without altering the nodes' values.
For example,
Given{1,2,3,4}, reorder it to{1,4,2,3}.
 1 package com.cn.cya.reorderlist;
 2  class ListNode {
 3           int val;
 4           ListNode next;
 5           ListNode(int x) {
 6               val = x;
 7               next = null;
 8           }
 9       }
10 public class Solution {
11     /**
12      * 1 首先将链表分成两份
13      * 2 将后面链表逆置
14      * 3 将两个链表拼起来
15      */
16     
17     public  void reorderList(ListNode head) {
18         ListNode haflist=findMiddle(head);
19         
20         ListNode reversehaflist=reverseList(haflist);
21         twolist(head,reversehaflist);
22     }
23     public ListNode findMiddle(ListNode head){//找到链表的中点，原理是 设置快慢指针，让快指针到底的时候，慢指针刚刚好在中间。这个应用有很多，比如找链表的第K个节点
24         if(head==null)return null;
25         ListNode fast=head;
26         ListNode slow=head;
27         while(fast!=null&&fast.next!=null){
28             slow=slow.next;
29             fast=fast.next.next;
30         }
31         ListNode tail=slow;
32         ListNode node=slow.next;
33         tail.next=null;
34         return node;
35         
36     }
37     public ListNode reverseList(ListNode list){//链表逆置
38         ListNode newHead=new ListNode(0);
39           ListNode p=null;
40         while(list!=null){
41             p=list.next;
42             list.next=newHead.next;
43             newHead.next=list;
44             list=p;
45         }
46         return newHead.next;
47     }
48     public  void twoMergeList(ListNode list1,ListNode list2){//将两个链表合并成一个链表list1，大师这个合并有问题，一直报错，在牛客网中的测试用例中一直都是25%通过，有朋友能帮我看下吗？
49         ListNode p1=list1;
50         ListNode p2=list2;
51         ListNode temp=null;
52         while(list1!=null&&list2!=null){
53             temp=p2.next;
54             p2.next=p1.next;
55             p1.next=p2;
56             p1=p1.next.next;
57             p2=temp;
58         }
59     }
60     /**
61      * 
62      * @param l1
63      * @param l2
64      * @return
65      */
66     public static ListNode merge(ListNode l1, ListNode l2){//这个合并是我借鉴的其他朋友滴
67         ListNode cursor1=l1, cursor2=l2;
68         ListNode dummy=new ListNode(-1);
69         ListNode cursor=dummy;
70         while(cursor1!=null&&cursor2!=null){
71             cursor.next=cursor1;
72             cursor1=cursor1.next;
73             cursor=cursor.next;
74             cursor.next=cursor2;
75             cursor2=cursor2.next;
76             cursor=cursor.next;
77         }
78         while(cursor1!=null){
79             cursor.next=cursor1;
80             cursor1=cursor1.next;
81             cursor=cursor.next;
82         }
83         while(cursor2!=null){
84             cursor.next=cursor2.next;
85             cursor2=cursor2.next;
86             cursor=cursor.next;
87         }
88          
89         return dummy.next;
90     }
91 }