pairSum = function (head) {
  const mid = llLength(head) / 2;
  const rightPointer = getRightPointer(head, mid);
  const leftPointer = reverseLL(head, mid);
  
  return getMax(leftPointer, rightPointer);
}
Linear Time 
Time O(n) | Space O(1)
@param {ListNode} head
@return {number}