Checks if a linked list is a palindrome by finding the middle, reversing the second half, and comparing.
## What is it?
Check whether a singly linked list is a palindrome — reads the same forwards and backwards. Achieved in O(n) time and O(1) space using the find-middle + reverse-half technique.
## How it works
1. Find the middle of the list (fast/slow pointers)
2. Reverse the second half of the list in-place
3. Compare the first half and the reversed second half node by node
4. (Optional) Restore the original list by reversing the second half back
## When to use
- Palindrome validation on a singly linked list where random access is unavailable
- Problems where you cannot use extra space (ruling out a stack or array approach)
## Key Points
- O(n) time, O(1) space — the key insight is to reverse in-place
- Destroying the list structure can be avoided by restoring after comparison
- Using a stack is simpler but O(n) space
