Mirror Pairs in a List

Medium

You get a singly linked list of even length n whose first node is first. Nodes are numbered from zero. Node i and node n - 1 - i form a *mirror pair*: the first with the last, the second with the second-to-last, and so on. The sum of a pair is the sum of its two node values. Return the largest sum over all mirror pairs.

Example 1

first=list_node([3, 8, 1, 6]) → 9

The pairs are the first and last nodes, 3 + 6 = 9, and the second and third, 8 + 1 = 9. The maximum is 9.

Example 2

first=list_node([2, 10, 4, 7, 1, 5]) → 11

The pairs are 2 + 5 = 7, 10 + 1 = 11 and 4 + 7 = 11. The answer is 11.

Example 3

first=list_node([50, 50]) → 100

Constraints

The number of nodes is even, between 2 and 10⁵
1 ≤ Node.val ≤ 10⁵

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Topics

Stack Linked List Two Pointers

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Input

first=list_node([3, 8, 1, 6])

Expected

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