public class Solution

{

    public ListNode OddEvenList(ListNode head)

    {

        if(head == null || head.next == null || head.next.next == null) return head;  //考虑特殊情况，0、1、2个点，直接返回

        ListNode res = head,a = head;          //a遍历奇数点  res记录奇数最开始的点

        ListNode bb=head.next,b = head.next;   //b遍历偶数点  bb 记录偶数最开始的点

        while (a.next != null && a.next.next !=null) {

            a.next = a.next.next;

            b.next = b.next.next;

            a = a.next;

            b = b.next;

        }

        if (a.next != null)

        {

            b.next = null;

        }

        a.next = bb;

        return res;

    }

}