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;
    }
};

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