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

/**
 * 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) {}
 * };
 */

// Time: O(n)
// Space: O(1)
class Solution {
public:
    int pairSum(ListNode* head) {
        ListNode *fast = head, *slow = head;

        while(fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        slow = reverseList(slow);
        
        int mx = 0;
        while(head && slow) {
            mx = max(mx, head->val + slow->val);
            head = head->next;
            slow = slow->next;
        }
        
        return mx;
    }
private:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = NULL;
        ListNode* curr = head;
        ListNode* NEXT = NULL;
        
        while(curr) {
            NEXT = curr->next;
            curr->next = prev;
            prev = curr;
            curr = NEXT;
        }
        
        return prev;
    }
};