Maximum twin sum of a linked list

Linked List Two Pointers Stack

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Feb 25, 2026

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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. 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.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
    let mut vals = Vec::new();
    let mut cur = &head;
    while let Some(node) = cur {
      vals.push(node.val);
      cur = &node.next;
    }
    let n = vals.len();
    (0..n / 2).map(|i| vals[i] + vals[n - 1 - i]).max().unwrap_or(0)
  }
}