Leetcode problem 100 - 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: Input: head = [5,4,2,1] 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: Input: head = [4,2,2,3] 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: Input: head = [1,100000] 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

/**
 * 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) {
  // Declare an array to store the values of the linked list nodes.
  const values = [];
  // Initialize a variable 'current' to the head of the linked list.
  let current = head;
  // Iterates through the linked list until 'current' becomes null.
  while (current) {
    // Push the value of the current node to the 'values' array.
    values.push(current.val);
    // Move 'current' to the next node in the linked list.
    current = current.next;
  }
  // Initialize a variable 'maxSum' to store the maximum twin sum.
  let maxSum = 0;
  // Get the length of the 'values' array and store it in the variable 'n'.
  const n = values.length;

  // Iterate through the first half of the 'values' array.
  for (let i = 0; i < n / 2; i++) {
    // Update the 'maxSum' variable if the current twin sum is greater than the previous maximum twin sum.
    maxSum = Math.max(maxSum, values[i] + values[n - 1 - i]);
  }
  // Return the calculated maximum twin sum.
  return maxSum;
};

Time Complexity: O(n), where n is the number of nodes in the linked list. Space Complexity: O(n), where n is the number of nodes in the linked list.