思路1：類似LeetCode86. 分隔連結串列，設定兩個虛擬頭節點，最後拼接即可。

☆☆思路2：分離節點後合併，不需要設定虛擬頭節點

class Solution {
    public ListNode oddEvenList(ListNode head) {
        if (head == null || head.next == null) return head;
        /**
         *  方法1：設定兩個虛擬頭節點
         */
        /*
        ListNode dummyHead1 = new ListNode(0);
        ListNode dummyHead2 = new ListNode(0);
        ListNode node1 = dummyHead1;
        ListNode node2 = dummyHead2;
        int cnt = 1;
        while (head != null) {
            if ((cnt & 1) == 1) {
                node1.next = head;
                node1 = node1.next;
            }else {
                node2.next = head;
                node2 = node2.next;
            }
            head = head.next;
            cnt ++;
        }
        node2.next = null;
        node1.next = dummyHead2.next;
        return dummyHead1.next;
        
 
*/
        /**
         * 方法2：不需要設定虛擬頭節點
         */
        ListNode evenHead = head.next; // 偶數連結串列的頭節點
        ListNode odd = head, even = head.next;
        while (even != null && even.next != null) {
            odd.next = even.next;
            odd = odd.next;
            even.next = odd.next;
            even 
 
= even.next;
        }
        odd.next = evenHead;
        return head;
    }
}