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+// 2130. Maximum Twin Sum of a Linked List
+#include "leetcode.h"
+
+/*
+ * in a linked-list of size 'n', where 'n' is even, the i'th node (0-indexed) of
+ * the linked list is known as the twin of the 'n - 1 - i' 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 node 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.
+ *
+ */
+
+class Solution {
+public:
+ int pairSum(ListNode *head) {
+ int ans = 0;
+ auto slow = head, fast = head;
+ while (fast != nullptr && fast->next != nullptr) {
+ fast = fast->next->next;
+ slow = slow->next;
+ }
+ slow = reverseList(slow);
+ while (slow != nullptr) {
+ ans = max(ans, slow->val + head->val);
+ slow = slow->next;
+ head = head->next;
+ }
+ return ans;
+ }
+
+private:
+ ListNode *reverseList(ListNode *curr, ListNode *prev = nullptr) {
+ while (curr != nullptr) {
+ swap(curr->next, prev);
+ swap(prev != nullptr ? prev->next : prev, curr);
+ }
+ return prev;
+ }
+};