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 {
    public void rotate(int[][] matrix) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(matrix==null) {
            return;
        }
        int n = matrix.length;
        for(int i=0;i<n/2;i++) {
            for(int j=0;j<Math.ceil(((double)n)/2.);j++) {
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[n-1-j][i];
                matrix[n-1-j][i] = matrix[n-1-i][n-1-j];
                matrix[n-1-i][n-1-j] = matrix[j][n-1-i];
                matrix[j][n-1-i] = tmp;
            }
        }
    }
}