// 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; } };