Rotate Image

Problem

Given an n × n 2D matrix representing an image, rotate the image 90° clockwise. Modify the matrix in place; do not allocate another matrix.

Constraints

• 1 ≤ n ≤ 20

Approach — Core Patterns

This is a Core Patterns problem. The idea: pick the data structure best matched to the constraints and solve it in one clean pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: see the walkthrough below.

Solution code

Python

class Solution:
    def rotate(self, matrix):
        matrix.reverse()
        for i in range(len(matrix)):
            for j in range(i):
                matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
        return matrix

Java

class Solution {
    public void rotate(int[][] m) {
        int n = m.length;
        for (int i = 0; i < n; i++)
            for (int j = i + 1; j < n; j++) {
                int t = m[i][j]; m[i][j] = m[j][i]; m[j][i] = t;
            }
        for (int i = 0; i < n; i++)
            for (int l = 0, r = n - 1; l < r; l++, r--) {
                int t = m[i][l]; m[i][l] = m[i][r]; m[i][r] = t;
            }
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.