# 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`

/**
* 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 *pre = NULL;
ListNode *next = NULL;
while (curr != NULL)
{
next = curr->next;
curr->next = pre;
pre = curr;
curr = next;
}
return pre;
}

{
stack<int> st;
int ans = INT_MIN;
while (fast->next->next != NULL)
{
st.push(slow->val);
slow = slow->next;
fast = fast->next->next;
}
st.push(slow->val);
slow = slow->next;