Reverse List In Place

Problem

Reverse a singly linked list in place and return the new head. No extra list, no array copy; only pointer rewiring. Then do the same for every group of k nodes.

It is a warm-up, but I use it to see how you handle pointers under a little pressure and whether you test your own code before I do.

Examples

Example 1, 1 -> 2 -> 3 -> 4 -> null → 4 -> 3 -> 2 -> 1 -> null

Example 2, k = 2 on 1 -> 2 -> 3 -> 4 -> 5 → 2 -> 1 -> 4 -> 3 -> 5 The leftover group (size < k) stays as is.

Edge, empty list → empty; single node → itself.

Constraints

• 0 <= n <= 10^5 nodes.
• O(n) time, O(1) extra space for the basic version. The recursive version is allowed only if you explain stack depth.
• For the k-group variant, 1 <= k <= n.

What they look for

The three-pointer walk (prev, cur, next) written without a bug on the first try, and drawing it on paper before typing. For k groups: counting ahead to check the group is full, reversing it, and reconnecting the tail of the previous group. In C, I also ask what happens if you forget to NULL the old head's next.