In a linked list of size n
, where n
is even, the i^{th}
node (0indexed) of the linked list is known as the twin of the (n1i)^{th}
node, if 0 <= i <= (n / 2)  1
.
 For example, if
n = 4
, then node0
is the twin of node3
, and node1
is the twin of node2
. These are the only nodes with twins forn = 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:
(5) > [4] > [2] > (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:
(4) > [2] > [2] > (3)
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:
(1) > (100000)
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, 10^{5}]
. 1 <= Node.val <= 10^{5}

