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23. Rotate Array

easyAsked at Plaid

Rotate an array to the right by k steps in-place. Plaid asks this because shifting a circular buffer of recent transactions by a rotation amount has the same shape.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • Glassdoor (2025)— Plaid SWE I OA.
  • LeetCode Discuss (2026)— Plaid intro.

Problem

Given an integer array nums, rotate the array to the right by k steps, where k is non-negative. Follow up: try doing 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. Rotate one step at a time, k times

Pop the last element and unshift it onto the front, k times.

Time
O(n*k)
Space
O(1)
function rotate(nums, k) {
  k %= nums.length;
  for (let i = 0; i < k; i++) {
    nums.unshift(nums.pop());
  }
}

Tradeoff: TLE for large k. Don't ship this. Also unshift is O(n).

2. Triple reverse in-place

Reverse the entire array, then reverse [0..k-1], then reverse [k..n-1].

Time
O(n)
Space
O(1)
function rotate(nums, k) {
  k %= nums.length;
  function rev(lo, hi) {
    while (lo < hi) { [nums[lo], nums[hi]] = [nums[hi], nums[lo]]; lo++; hi--; }
  }
  rev(0, nums.length - 1);
  rev(0, k - 1);
  rev(k, nums.length - 1);
}

Tradeoff: Three reversals = linear time, in-place. The k %= n step is critical or large k still loops past the end.

Plaid-specific tips

Plaid grades this on the triple-reverse trick because it shows you've internalized the in-place rotation pattern. Bonus signal: derive why three reversals work — reversing the whole then reversing the two halves restores order inside each block while swapping their positions.

Common mistakes

  • Forgetting k %= n — pointless work or out-of-bounds.
  • Allocating a new array — fails the in-place constraint.
  • Using unshift in a loop — each unshift is O(n).

Follow-up questions

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

  • Rotate by k positions to the left instead.
  • Rotate a 2D matrix by 90 degrees (LC 48).
  • Rotate a singly-linked list (LC 61).

Solve it now

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Output

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FAQ

Why does triple-reverse work?

After reversing the whole array, the last k elements move to the front but in reverse order. Reversing each block restores their original order, placing them at the front.

Why does k %= n matter?

k larger than n is the same rotation as k mod n. Without the mod, you'd do extra wasted passes.

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