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}.

package com.cn.cya.reorderlist;
 class ListNode {
          int val;
          ListNode next;
          ListNode(int x) {
              val = x;
              next = null;
          }
      }
public class Solution {
    /**
     * 1 首先将链表分成两份
     * 2 将后面链表逆置
     * 3 将两个链表拼起来
     */
    
    public  void reorderList(ListNode head) {
        ListNode haflist=findMiddle(head);
        
        ListNode reversehaflist=reverseList(haflist);
        twolist(head,reversehaflist);
    }
    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
        if(head==null)return null;
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
        }
        ListNode tail=slow;
        ListNode node=slow.next;
        tail.next=null;
        return node;
        
    }
    public ListNode reverseList(ListNode list){//链表逆置
        ListNode newHead=new ListNode(0);
          ListNode p=null;
        while(list!=null){
            p=list.next;
            list.next=newHead.next;
            newHead.next=list;
            list=p;
        }
        return newHead.next;
    }
    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？
        ListNode p1=list1;
        ListNode p2=list2;
        ListNode temp=null;
        while(list1!=null&&list2!=null){
            temp=p2.next;
            p2.next=p1.next;
            p1.next=p2;
            p1=p1.next.next;
            p2=temp;
        }
    }
    /**
     * 
     * @param l1
     * @param l2
     * @return
     */
    public static ListNode merge(ListNode l1, ListNode l2){//这个合并是我借鉴的其他朋友滴
        ListNode cursor1=l1, cursor2=l2;
        ListNode dummy=new ListNode(-1);
        ListNode cursor=dummy;
        while(cursor1!=null&&cursor2!=null){
            cursor.next=cursor1;
            cursor1=cursor1.next;
            cursor=cursor.next;
            cursor.next=cursor2;
            cursor2=cursor2.next;
            cursor=cursor.next;
        }
        while(cursor1!=null){
            cursor.next=cursor1;
            cursor1=cursor1.next;
            cursor=cursor.next;
        }
        while(cursor2!=null){
            cursor.next=cursor2.next;
            cursor2=cursor2.next;
            cursor=cursor.next;
        }
         
        return dummy.next;
    }
}