Formatted question description: https://leetcode.ca/all/2130.html

# 2130. Maximum Twin Sum of a Linked List (Medium)

In a linked list of size `n`

, where `n`

is **even**, the `i`

node (^{th}**0-indexed**) of the linked list is known as the **twin** of the `(n-1-i)`

node, if ^{th}`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:6Explanation: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:7Explanation: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:100001Explanation: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, 10`

.^{5}] `1 <= Node.val <= 10`

^{5}

**Related Topics**:

Linked List, Two Pointers, Stack

**Similar Questions**:

## Solution 1.

```
// OJ: https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list/
// Time: O(N)
// Space: O(N)
class Solution {
int getLength(ListNode *head) {
int ans = 0;
for (; head; head = head->next) ++ans;
return ans;
}
public:
int pairSum(ListNode* head) {
int len = getLength(head), ans = 0;
vector<int> sum(len / 2);
for (int i = 0; head; head = head->next, ++i) {
ans = max(ans, sum[i < len / 2 ? i : (len - 1 - i)] += head->val);
}
return ans;
}
};
```

## Solution 2.

```
// OJ: https://leetcode.com/problems/maximum-twin-sum-of-a-linked-list/
// Time: O(N)
// Space: O(1)
class Solution {
ListNode *reverseList(ListNode *head) {
ListNode dummy;
while (head) {
auto p = head;
head = head->next;
p->next = dummy.next;
dummy.next = p;
}
return dummy.next;
}
public:
int pairSum(ListNode* head) {
auto p = head, q = head;
while (q->next && q->next->next) {
p = p->next;
q = q->next->next;
}
q = p->next;
p->next = nullptr;
q = reverseList(q);
int ans = 0;
for (; head; head = head->next, q = q->next) {
ans = max(ans, head->val + q->val);
}
return ans;
}
};
```