Check If the Singly Linked List is a Palindrome.

The list shows above is a palindrome and we need to check if it is indeed a palindrome. Among several approaches, the following is a simple and easy to understand method.

Algorithm:
1. Get the middle of the linked list.
2. Reverse the second half of the linked list.
3. Compare the first half and second half.
4. Construct the original linked list by reversing the
second half again and attaching it back to the first half

Time Complexity O(n)
Space Complexity O(1)