給一個 linked list，要以 偶數位 奇數位 重新排列。
要求在 O(1) Space、 O(n) Time 解決。
分別拆成偶數奇數頭出來，再把剩下的 linked list 分給兩者，最後合併，回傳偶數頭。這符合題目要求。
或者直接在頭尾開始接，但這預期不如上者，因為上者到 n-1 位置時，就幾乎要完成了；而這方法才剛開始。
Python
class Solution:
def oddEvenList(self, head: ListNode) -> ListNode:
if not head or not head.next: return head
oddHead = OP = head
evenHead = EP = head.next
#
head = head.next.next
i = 0
while head:
i += 1
if i % 2 == 1:
OP.next = head
OP = OP.next
else:
EP.next = head
EP = EP.next
head = head.next
EP.next = None
OP.next = evenHead
return oddHead
C#
public class Solution {
public ListNode OddEvenList(ListNode head) {
if(head is null){return head;}
if(head.next is null){return head;}
ListNode OddHead = head; ListNode OddLN = OddHead;
ListNode EvenHead = OddHead.next; ListNode EvenLN = EvenHead;
head = head.next.next;
int i = 0;
while(!(head is null))
{
if(i % 2 == 0)
{
OddLN.next = head;
OddLN = OddLN.next;
}
else
{
EvenLN.next = head;
EvenLN = EvenLN.next;
}
i += 1;
head = head.next;
}
EvenLN.next = null;
OddLN.next = EvenHead;
return OddHead;
}
}