2130-maximum-twin-sum-of-a-linked-list.js

/**
 * 2130. Maximum Twin Sum of a Linked List
 * https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list/
 * Difficulty: Medium
 *
 * 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.
 */

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {number}
 */
var pairSum = function(head) {
  const stack = [];
  while (head) {
    stack.push(head.val);
    head = head.next;
  }

  let max = 0;
  for (let i = 0; i < stack.length; i++) {
    max = Math.max(max, stack[i] + stack[stack.length - 1 - i]);
  }

  return max;
};