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Abhishek Chaudhary

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Maximum Twin Sum of a Linked List

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`

SOLUTION:

``````# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
return 0

def pairSum(self, head: Optional[ListNode]) -> int:
sums = {}