Leetcode - Rotate Image

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) {
        if(matrix == null)  return;
        int n = matrix.length;
        // n * n matrix
        //int w = matrix[0].length;
        if(n < 2) return;
        
        // rule: i = j'; j = n-1 - i';
        // that is: i' = n-1 -j; j' = i;
        // loop through 1/4 of the matrix
        // a | b
        // c | d
        // four parts
        // Math.ceil : returns the smallest integer >= a number.
        for(int i = 0; i < n/2; i++) {
            for(int j = 0; j < Math.ceil(((double)n)/2.0); j++) {
                int tmp = matrix[i][j];
                
                // c -> a
                matrix[i][j] = matrix[n-1 - j][i];
                // d -> c
                matrix[n-1 - j][i] = matrix[n-1 - i][n-1 - j];
                // b -> d
                matrix[n-1 - i][n-1 - j] = matrix[j][n-1 - i];
                // a -> b (tmp -> b)
                matrix[j][n-1 - i] = tmp;
            }
        }
    }
}