leetcode--Rotate Image

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?

public class Solution {
    /**The algorithm is simple.
     * @author Averill Zheng
     * @version 2014-06-15
     * @since JDK 1.7
     */
    public void rotate(int[][] matrix) {
        int length = matrix.length;
		for(int i = 0; i < length / 2; ++i){
			for(int k = i; k < length - 1 - i; ++k){
				matrix[i][k] += (matrix[k][length - 1 - i] + matrix[length - 1 - i][length - 1 - k] 
								 + matrix[length - 1 - k][i]);
				matrix[k][length - 1 - i] = (matrix[i][k] - matrix[k][length - 1 - i] 
											 - matrix[length - 1 - i][length - 1 - k] 
											 - matrix[length - 1 - k][i]);
				matrix[length - 1 - i][length - 1 - k] = (matrix[i][k] - matrix[k][length - 1 - i] 
						 								  - matrix[length - 1 - i][length - 1 - k] 
						 								  - matrix[length - 1 - k][i]);
				matrix[length - 1 - k][i] = (matrix[i][k] - matrix[k][length - 1 - i] 
						   					 - matrix[length - 1 - i][length - 1 - k] 
						   					 - matrix[length - 1 - k][i]);
				matrix[i][k] = (matrix[i][k] - matrix[k][length - 1 - i] 
						        - matrix[length - 1 - i][length - 1 - k] 
						        - matrix[length - 1 - k][i]);
			}
		}    
    }
}