2130. 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.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
    ListNode* reverse(ListNode* head) {
        ListNode* dummy = new ListNode(-1);
        while(head) {
            auto nxt = head->next;
            head->next = dummy->next;
            dummy->next = head;
            head = nxt;
        }
        
        return dummy->next;
    }
public:
    int pairSum(ListNode* head) {
        ListNode *fast = head, *slow = head;
        while(fast) {
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* second = reverse(slow);
        int res = 0;
        while(second) {
            res = max(res, second->val + head->val);
            second = second->next;
            head = head->next;
        }
        return res;
    }
};