2130. Maximum Twin Sum of a Linked List

Problem

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

Solution

/**
 * 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) {
    var num = 0;
    var node = head;
    while (node) {
        num++;
        node = node.next;
    }

    var stack = [];
    var sum = 0;
    var i = 0;
    while (i < num) {
        if (i < num / 2) {
            stack.push(head.val);
        } else {
            sum = Math.max(sum, head.val + stack.pop());
        }
        i += 1;
        head = head.next;
    }

    return sum;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).