How would you check if a singly linked list is a palindrome with O(1) extra space?

• A. Use a queue to compare symmetric nodes without extra space.
• B. Reverse the second half in place and compare halves.
• C. Copy the list into a stack to compare.
• D. Use a hash table to store first half values.

Answer: (D)

To check a singly linked list for palindrome with constant extra space, the trick is to avoid storing nodes and instead compare mirrored positions by altering the list temporarily. Locate the middle of the list using a slow and a fast pointer. Then reverse the second half in place, changing next pointers rather than creating new nodes. With one pointer at the head and another at the start of this reversed half, walk forward and compare the node values pair by pair. If all pairs match (and you skip the middle node for odd-length lists), the list is a palindrome. Optionally reverse the second half again to restore the original list. This approach uses only a few pointers, giving O(n) time and O(1) extra space. Using a queue, a stack, or a hash table would store part of the list and thus require O(n) extra space, which is unsuitable when constant space is the goal.