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24. Rotate Array

easyAsked at Reddit

Rotate an array to the right by k steps. Reddit uses the reverse-then-reverse trick to gauge in-place manipulation — the same skill needed to rotate a fixed-size sliding ranking window without re-allocating.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit infra-team phone screen.

Problem

Given an integer array nums, rotate the array to the right by k steps, where k is non-negative. Try to come up with as many solutions as you can. There are at least three different ways to solve this problem. Could you do it in-place with O(1) extra space?

Constraints

  • 1 <= nums.length <= 10^5
  • -2^31 <= nums[i] <= 2^31 - 1
  • 0 <= k <= 10^5

Examples

Example 1

Input
nums = [1,2,3,4,5,6,7], k = 3
Output
[5,6,7,1,2,3,4]

Example 2

Input
nums = [-1,-100,3,99], k = 2
Output
[3,99,-1,-100]

Approaches

1. Use extra array

Copy nums[i] to result[(i + k) % n].

Time
O(n)
Space
O(n)
function rotate(nums, k) {
  const n = nums.length;
  k = k % n;
  const out = new Array(n);
  for (let i = 0; i < n; i++) out[(i + k) % n] = nums[i];
  for (let i = 0; i < n; i++) nums[i] = out[i];
}

Tradeoff: Linear but uses O(n) memory. Doesn't satisfy the O(1) follow-up.

2. Reverse three times (optimal)

Reverse the whole array. Reverse the first k. Reverse the rest.

Time
O(n)
Space
O(1)
function rotate(nums, k) {
  const n = nums.length;
  k = k % n;
  const reverse = (l, r) => {
    while (l < r) {
      [nums[l], nums[r]] = [nums[r], nums[l]];
      l++; r--;
    }
  };
  reverse(0, n - 1);
  reverse(0, k - 1);
  reverse(k, n - 1);
}

Tradeoff: O(1) space. The triple-reverse trick is the classic Reddit-favored answer.

Reddit-specific tips

Reddit interviewers want the reverse trick. Bonus signal: explain WHY it works — reversing the whole flips order, reversing the first k re-orders within the front, reversing the back re-orders within the back. Same shape as their ranked-feed window-shift.

Common mistakes

  • Forgetting k = k % n (handles k > n).
  • Reversing the wrong segments (off-by-one).
  • Using slice/concat — O(n) memory.

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • Cyclic replacements (k = gcd(n,k) cycles).
  • Rotate a linked list (LC 61).
  • Rotate a matrix (LC 48).

Solve it now

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Output

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FAQ

Why does the triple-reverse work?

If you label the original as [A B] where B has length k, the target is [B A]. Reverse whole = [B' A']. Reverse first k = [B A']. Reverse rest = [B A].

What if k > n?

k = k % n. Rotating by n is identity.

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