https://leetcode.com/problems/odd-even-linked-list/description/
Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.

You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.

Example:
Given 1->2->3->4->5->NULL,
return 1->3->5->2->4->NULL.

Note:
The relative order inside both the even and odd groups should remain as it was in the input.
The first node is considered odd, the second node even and so on ...

 1  public ListNode oddEvenList(ListNode head) {
 2         if (head == null || head.next == null) return head;
 3         ListNode oddCurr = head;
 4         ListNode evenHead = head.next ;
 5         ListNode evenCurr = head.next;
 6         /*
 7          1->2->3->4->5->NULL
 8          co ce
 
 9             eh
10          1->3     2->4
11               ->
12          * */
13         int count = 0;
14         /*
15         even is always one node ahead so:
16          evenCurr != null == oddCurr.next != null
17          evenCurr.next != null == oddCurr.next.next != null
18 
19         重要！when evenCurr is at 4, evenCurr.next = 5 != null
 
20         */
21         while (evenCurr != null && evenCurr.next != null){
22             oddCurr.next = oddCurr.next.next ;
23             oddCurr = oddCurr.next;
24             //4->5.next=null 4->null
25             evenCurr.next = evenCurr.next.next ;
26             evenCurr = evenHead.next;
27         }
28         //奇數結尾指向偶數的頭
29         oddCurr.next = evenHead ;
30         return head;
31     }

LC_328. Odd Even Linked List