2130. Maximum Twin Sum of a Linked List | LeetCode Solution

 2130. Maximum Twin Sum of a Linked List | LeetCode Solution

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.

 * 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

{

public:

    ListNode *reverseLinkedList(ListNode *head)

    {

        if (head == NULL)

            return head;

        ListNode *pre = NULL;

        ListNode *curr = head;

        ListNode *next = NULL;

        while (curr != NULL)

        {

            next = curr->next;

            curr->next = pre;

            pre = curr;

            curr = next;

        }

        return pre;

    }

    int pairSum(ListNode *head)

    {

        stack<int> st;

        int ans = INT_MIN;

        ListNode *fast = head, *slow = head;

        while (fast->next->next != NULL)

        {

            st.push(slow->val);

            slow = slow->next;

            fast = fast->next->next;

        }

        st.push(slow->val);

        slow->next = reverseLinkedList(slow->next);

        slow = slow->next;

        ListNode *curr = head;

        while (slow != NULL)

        {

            ans = max(curr->val + slow->val, ans);

            slow = slow->next;

            curr = curr->next;

        }

        return ans;

    }

};