# 2130. Maximum Twin Sum of a Linked List

Medium
In a linked list of size `n`, where `n` is even, the `ith` node (0-indexed) of the linked list is known as the twin of the `(n-1-i)th` node, if `0 <= i <= (n / 2) - 1`.
• For example, if `n = 4`, then node `0` is the twin of node `3`, and node `1` is the twin of node `2`. These are the only nodes with twins for `n = 4`.
The twin sum is defined as the sum of a node and its twin.
Given the `head` of a linked list with even length, return the maximum twin sum of the linked list.
Example 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: Output:
7
Explanation:
The nodes with twins present in this linked list are:
- Node 0 is the twin of node 3 having a twin sum of 4 + 3 = 7.
- Node 1 is the twin of node 2 having a twin sum of 2 + 2 = 4.
Thus, the maximum twin sum of the linked list is max(7, 4) = 7.
Example 3: Output:
100001
Explanation:
There is only one node with a twin in the linked list having twin sum of 1 + 100000 = 100001.
Constraints:
• The number of nodes in the list is an even integer in the range `[2, 105]`.
• `1 <= Node.val <= 105`

### 解題

/**
* type ListNode struct {
* Val int
* Next *ListNode
* }
*/