How do you approach Rotate Image?

First transpose the matrix (swap rows and columns).

What is the time complexity of Rotate Image?

Time complexity: O(n²). Space complexity: O(1).

What are common mistakes on Rotate Image?

Transposing row and column in the index. It reads fine and fails on any non-square input. Overwriting cells a later iteration still needs to read - most in-place matrix problems want a marker pass first. Bounds checks that cover only some of the four edges.

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Rotate Image

MediumAlgorithms

Description

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]

Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2

Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]

Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

Constraints

- n == matrix.length == matrix[i].length - 1 <= n <= 20 - -1000 <= matrix[i][j] <= 1000

Rotate Image: approach, complexity and pitfalls

A medium matrix problem, graded against 7 test cases (4 of them hidden).

The pattern: Matrix

2D grid traversal, rotation, and in-place manipulation.

Reach for it when you see: A grid input, spiral or diagonal traversal, or an in-place transformation constraint.

If you are stuck

Show the 3-step hint ladder

First transpose the matrix (swap rows and columns).
Then reverse each row.
This achieves a 90-degree clockwise rotation.

The optimal approach

Show the approach: Transpose + Reverse

1. Transpose the matrix (swap matrix[i][j] with matrix[j][i])

2. Reverse each row

Time: O(n²)
Space: O(1)

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Single element. A one-element case is included. Two-pointer and window solutions frequently return the wrong thing when the two indices start out equal.
Duplicate values. Repeated values appear in the input, so any logic that assumes distinct elements - or that skips over equal neighbours - is exercised.
Negative numbers. The suite includes negative values. Running-sum, min/max seeding and any "initialise to zero" step need to handle them.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Matrix mistakes

These apply to the pattern as a whole, not just this problem.

Transposing row and column in the index. It reads fine and fails on any non-square input.
Overwriting cells a later iteration still needs to read - most in-place matrix problems want a marker pass first.
Bounds checks that cover only some of the four edges.