https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list/description/
2130. Maximum Twin Sum of a Linked List
給你一個偶數數量的鏈結串列，返回最大的一對串列和，一對串列為串列對半切之後從
中心點分別往左和右有相同距離的Node。
Example 1:
https://assets.leetcode.com/uploads/2021/12/03/eg1drawio.png
Input: head = [5,4,2,1]
Output: 6
Explanation:
Nodes 0 and 1 are the twins of nodes 3 and 2, respectively. All have twin sum
= 6.
There are no other nodes with twins in the linked list.
Thus, the maximum twin sum of the linked list is 6.
Example 2:
https://assets.leetcode.com/uploads/2021/12/03/eg3drawio.png
Input: head = [1,100000]
Output: 100001
Explanation:
There is only one node with a twin in the linked list having twin sum of 1 +
100000 = 100001.
思路：
1.一個指標每次走一步，一個每次走兩步，當快指標走完的時候慢指標會在中間，把
慢指標停留的值紀錄在一個List。
2.慢指標繼續一步一步往後走並和 List 的尾端元素開始往前相加，並更新最大值。
Java Code：