Rotate Image

ID: 48

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.

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

Idea

Pure matrix manipulation, index trick

For a odd size matrix (3x3), only first two row and first one column should be considered, other indexes can be calculated based on these info.

For a even size matrix(4x4), first two row and first two column should be considered.

So, i in range(0, (n+1)/2), j in range(0. n/2)

Code

public void rotate(int[][] matrix) {
        int n = matrix.length;
        for(int i = 0; i<(n+1)/2; i++){
            for(int j = 0; j<n/2; j++){
                int temp = matrix[n - 1 - j][i];
                matrix[n - 1 - j][i] = matrix[n - 1 - i][n - j - 1];
                matrix[n - 1 - i][n - j - 1] = matrix[j][n - 1 -i];
                matrix[j][n - 1 - i] = matrix[i][j];
                matrix[i][j] = temp;
            }
        }
    }