2130. Maximum Twin Sum of a Linked List

Solution

class ListNode {
  val: number;
  next: ListNode | null;
  constructor(val?: number, next?: ListNode | null) {
    this.val = val === undefined ? 0 : val;
    this.next = next === undefined ? null : next;
  }
}

function pairSum(head: ListNode | null): number {
  if (head?.next && !head?.next?.next) {
    return head.val + head.next.val;
  }

  const arr = [];
  let n = 0;
  let currentNode = head;

  while (currentNode) {
    n++;
    arr.push(currentNode.val);
    currentNode = currentNode.next;
  }

  let twinSum = 0;

  for (let i = 0; i < arr.length / 2; i++) {
    twinSum = Math.max(twinSum, arr[i] + arr[n - 1 - i]);
  }

  return twinSum;
}

const first = new ListNode(5);
const second = new ListNode(4);
const third = new ListNode(2);
const fourth = new ListNode(1);

first.next = second;
second.next = third;
third.next = fourth;

pairSum(first);