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2130. Maximum Twin Sum of a Linked List

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Problem Statement

2130. Maximum Twin Sum of a Linked List

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.

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:

Java

Source file
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public int pairSum(ListNode head) {
        // Find head of latter half (i.e., middle node)
        ListNode mid = findMiddle(head);
        // Reverse latter half of LL (new head is the last node of the OG LL)
        ListNode newHead = reverseLL(mid);
        // print(head); // note this will have 1 extra node (which is the mid), disregard it
        // print(newHead);
        int ans = Integer.MIN_VALUE;
        while (newHead != null) {
            ans = Math.max(ans, newHead.val + head.val);
            newHead = newHead.next;
            head = head.next;
        }
        return ans;
    }

    // void print(ListNode head) {
    //     while (head != null) {
    //         System.out.print(head.val + "->");
    //         head = head.next;
    //     }
    // }

    ListNode findMiddle(ListNode head) {
        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    ListNode reverseLL(ListNode head) {
        ListNode curr = head, prev = null, temp;
        while (curr != null) {
            temp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = temp;
        }
        return prev;
    }
}