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61. Rotate List

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Rotate a singly linked list to the right by k positions. The trick is realizing you don't move k nodes — you find the new tail with a single split, then close and reopen the loop. Modular-arithmetic trap when k exceeds the list length.

By Sam K., Founder, InterviewChamp.AI · Last verified

Problem

Given the head of a linked list, rotate the list to the right by k places.

Constraints

  • The number of nodes in the list is in the range [0, 500].
  • -100 <= Node.val <= 100
  • 0 <= k <= 2 * 10^9

Examples

Example 1

Input
head = [1,2,3,4,5], k = 2
Output
[4,5,1,2,3]

Example 2

Input
head = [0,1,2], k = 4
Output
[2,0,1]

Explanation: k = 4 is equivalent to k = 1 because the list has length 3.

Example 3

Input
head = [], k = 0
Output
[]

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Output

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Hints

Progressive — try the first before opening the next.

Hint 1

If k is larger than the list length, most of those rotations cancel out. Take k modulo length first.

Hint 2

Find the original tail, connect tail.next = head to form a temporary cycle.

Hint 3

Walk length − (k % length) − 1 steps from head to land on the new tail. Set newHead = newTail.next, break the cycle by newTail.next = null.

Solution approach

Reveal approach

Close-the-loop, then split. Edge-case: if head is null or k == 0, return head. Walk once to find the tail and the length n. Compute k = k % n; if k == 0, return head (no rotation needed). Close the list into a cycle: tail.next = head. Walk n − k − 1 steps from head to find the new tail. Set newHead = newTail.next, then newTail.next = null. Return newHead. Why this works: rotating right by k is the same as cutting the list at position n − k from the start and stitching the back half to the front. The temporary cycle saves you from juggling two list halves. O(n) time, O(1) extra space.

Complexity

Time
O(n)
Space
O(1)

Related patterns

  • linked-list
  • pointer-rewiring
  • two-pointers

Related problems

Asked at

Companies reported asking this problem (sourced from public Glassdoor, Blind, and Levels.fyi interview posts).

  • Amazon
  • Microsoft
  • Bloomberg

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