PPractick

The problem

A carousel on a shopping app stores its banners in a singly linked list, and pressing "skip ahead" should rotate the list to the right by k places: the last k banners move to the front, in order. k can be far larger than the list length, because users hold the button, so rotating one step at a time is out. The efficient trick is to temporarily close the list into a ring and then cut it at the right spot.

Example: 1 -> 2 -> 3 -> 4 -> 5, k = 2 → 4 -> 5 -> 1 -> 2 -> 3.

Function signature

Node* rotateRight(Node* head, int k);  // k >= 0; returns the new head

Test cases

Input (list, k)	Output list	Explanation
(1 -> 2 -> 3 -> 4 -> 5, 2)	4 -> 5 -> 1 -> 2 -> 3
(0 -> 1 -> 2, 4)	2 -> 0 -> 1	4 mod 3 = 1
(1 -> 2 -> 3, 3)	1 -> 2 -> 3	A full rotation changes nothing
(1, 99)	1
(empty, 5)	(empty)

(Confirm: rotation is to the right; k may exceed the length, up to about 2·10⁹; only links change.)

How interviewers phrase it

• "Given the head of a linked list, rotate the list to the right by k places" (LeetCode 61).
• Some versions rotate left (counter-clockwise), which means the first k nodes move to the end.

Clarifying questions to ask

1. Rotate right or left?
2. Can k be larger than the length? (Yes, so reduce it modulo n.)
3. How should empty lists be handled?

Follow-ups to expect

1. "Why k mod n?"
2. "Where do you cut?"
3. "Why connect the tail to the head?"
4. "How would a left rotation by k change it?"
5. "Complexity?"

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