2130 - Maximum Twin Sum of a Linked List\.

Medium

In a linked list of size n, where n is even , the <code>i^th</code> node ( 0-indexed ) of the linked list is known as the twin of the <code>(n-1-i)^th</code> 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 <code>2, 10⁵</code>.

• <code>1 <= Node.val <= 10⁵</code>