Rotate Image

Problem

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.

Examples

The brute-force approach

Create a copy of the matrix. For each cell (r, c), place it at position (c, n-1-r) in the result.

copy = [row[:] for row in matrix]
for r in range(n):
    for c in range(n):
        matrix[c][n-1-r] = copy[r][c]

O(n²) time and O(n²) space for the copy.

Spotting the pattern

This is a Matrix Traversal problem. The key question to ask yourself:

Why does transposing and then reversing each row produce a 90-degree clockwise rotation? What would you do for counter-clockwise instead?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.