分为以下几个步骤，1使用快慢指针找到中间节点，2翻转后半部分，3逐个插入节点。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public void reorderList(ListNode head) {
if(head==null||head.next==null)return;
ListNode slow = head ;
ListNode fast = head;
while(fast!=null)
{
slow=slow.next;
fast=fast.next==null?null:fast.next.next;
}
ListNode p1 =slow ;
ListNode p2 =slow.next;
p1.next=null;
while(p2!=null)
{
ListNode temp =p2.next;
p2.next=p1;
p1=p2;
p2=temp;
}
ListNode pos = head ;
while(p1!=null)
{
ListNode res1 = p1.next;
ListNode res2 = pos.next;
pos.next= p1;
p1.next=res2;
p1=res1;
pos=res2;
}
pos.next=null;
}
}
