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:

    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:

    Input: head = [4,2,2,3]
    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:

    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.

Constraints:

- The number of nodes in the list is an even integer in the range `[2, 105]`.
- `1 <= Node.val <= 105`