← Microsoft Interview Insights
The naive approach with a stack or array is obvious and I think they expected me to start there, but the real question is the in-place version.
First, clarify the constraints and confirm that O(1) space means we cannot use a stack or array. Then, describe the optimal approach: find the middle of the list using slow and fast pointers, reverse the second half in-place, compare the two halves, and finally restore the list to its original order. This achieves O(n) time and O(1) space.
Pro tip: Mention that you would restore the list to its original state after checking, as good practice to avoid side effects, and discuss edge cases like empty list, single node, and even/odd lengths.
Confirm that the list is singly linked, and that O(1) extra space means we cannot use additional data structures like arrays or stacks. Also, clarify if modifying the list is allowed (usually yes, but should be restored).
Use the slow and fast pointer technique: slow moves one step, fast moves two steps. When fast reaches the end, slow is at the middle. For even length, slow will be at the start of the second half.
Reverse the linked list starting from the slow pointer (or slow.next for odd length) to the end. This can be done iteratively with three pointers: prev, current, and next.
Traverse from the head and from the reversed second half simultaneously, comparing node values. If all match, it's a palindrome; otherwise, it's not.
Reverse the second half again to restore the original list structure, then return the boolean result. This step is optional but recommended for good practice.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.