message: Update maximum-twin-sum-of-a-linked-list.cpp
diff: // Time: O(n)
  // Space: O(1)
  -/**
  - * Definition for singly-linked list.
  - * 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:
  int pairSum(ListNode* head) {
  