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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 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

Related Topics:

Similar Questions:

## Solution 1.

• /**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode() {}
*     ListNode(int val) { this.val = val; }
*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
ListNode q = slow.next;
slow.next = null;
ListNode pb = reverse(q);
int ans = 0;
while (pa != null) {
ans = Math.max(ans, pa.val + pb.val);
pa = pa.next;
pb = pb.next;
}
return ans;
}

ListNode dummy = new ListNode();
while (curr != null) {
ListNode next = curr.next;
curr.next = dummy.next;
dummy.next = curr;
curr = next;
}
return dummy.next;
}
}

• /**
* 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:
while (fast && fast->next) {
slow = slow->next;
fast = fast->next->next;
}
ListNode* q = slow->next;
slow->next = nullptr;
ListNode* pb = reverse(q);
int ans = 0;
while (pa) {
ans = max(ans, pa->val + pb->val);
pa = pa->next;
pb = pb->next;
}
return ans;
}

ListNode* dummy = new ListNode();
while (curr) {
ListNode* next = curr->next;
curr->next = dummy->next;
dummy->next = curr;
curr = next;
}
return dummy->next;
}
};

• # Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
def pairSum(self, head: Optional[ListNode]) -> int:
dummy = ListNode()
while curr:
next = curr.next
curr.next = dummy.next
dummy.next = curr
curr = next
return dummy.next

while fast and fast.next:
slow, fast = slow.next, fast.next.next
q = slow.next
slow.next = None
pb = reverse(q)
ans = 0
while pa and pb:
ans = max(ans, pa.val + pb.val)
pa = pa.next
pb = pb.next
return ans


• /**
* type ListNode struct {
*     Val int
*     Next *ListNode
* }
*/
reverse := func(head *ListNode) *ListNode {
dummy := &ListNode{}
for curr != nil {
next := curr.Next
curr.Next = dummy.Next
dummy.Next = curr
curr = next
}
return dummy.Next
}
for fast != nil && fast.Next != nil {
slow, fast = slow.Next, fast.Next.Next
}
q := slow.Next
slow.Next = nil
pb := reverse(q)
ans := 0
for pa != nil {
ans = max(ans, pa.Val+pb.Val)
pa = pa.Next
pb = pb.Next
}
return ans
}

func max(a, b int) int {
if a > b {
return a
}
return b
}

• /**
* class ListNode {
*     val: number
*     next: ListNode | null
*     constructor(val?: number, next?: ListNode | null) {
*         this.val = (val===undefined ? 0 : val)
*         this.next = (next===undefined ? null : next)
*     }
* }
*/

function pairSum(head: ListNode | null): number {
while (fast) {
fast = fast.next.next;
slow = slow.next;
}
let prev = null;
while (slow) {
const next = slow.next;
slow.next = prev;
prev = slow;
slow = next;
}
let right = prev;
let ans = 0;
while (left && right) {
ans = Math.max(ans, left.val + right.val);
left = left.next;
right = right.next;
}
return ans;
}


• // Definition for singly-linked list.
// #[derive(PartialEq, Eq, Clone, Debug)]
// pub struct ListNode {
//   pub val: i32,
//   pub next: Option<Box<ListNode>>
// }
//
// impl ListNode {
//   #[inline]
//   fn new(val: i32) -> Self {
//     ListNode {
//       next: None,
//       val
//     }
//   }
// }
impl Solution {
pub fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
let mut arr = Vec::new();
while node.is_some() {
let t = node.as_ref().unwrap();
arr.push(t.val);
node = &t.next;
}
let n = arr.len();
let mut ans = 0;
for i in 0..n >> 1 {
ans = ans.max(arr[i] + arr[n - 1 - i]);
}
ans
}
}