RotateImg.java

import java.util.*;

/**
 * You are given an n x n 2D matrix representing an image.
 * 
 * Rotate the image by 90 degrees (clockwise).
 * 
 * Follow up:
 * Could you do this in-place?
 * 
 * Tags: Array
 */
class RotateImg {
    public static void main(String[] args) {
        int[][] matrix = {{1, 2}, {4, 3}};
        new RotateImg().rotate(matrix);
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[i].length; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }
    
    /**
     * Get the length of matrix
     * Do level by level, each level edge by edge
     * In-place solutions overwrites original matrix
     */
    public void rotate(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return;
        int n = matrix.length;
        for (int i = 0; i < n / 2 ; i++) {
            for (int j = i; j < n - i - 1 ; j++) {
                int tmp = matrix[i][j]; // save in tmp var
                matrix[i][j] = matrix[n-j-1][i]; // first col
                matrix[n-j-1][i] = matrix[n-i-1][n-j-1]; // last row
                matrix[n-i-1][n-j-1] = matrix[j][n-i-1]; // last col
                matrix[j][n-i-1] = tmp;
            }
        }
    }
}