[LeetCode] Rotate Image

Problem:

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?

Solution:O(n^2)

public class Solution {
    public void rotate(int[][] matrix) {
 if (matrix == null || matrix.length == 0
     || matrix.length != matrix[0].length)
 return;

 int rowcount = matrix.length;
 int s = rowcount -1;
 for (int i = 0; i < rowcount / 2; i++) {
     for (int j = i; j < rowcount - i-1; j++) {
  int tmp = matrix[i][j];
  matrix[i][j] = matrix[s-j][i];
  matrix[s-j][i] = matrix[s-i][s-j];
  matrix[s-i][s-j] = matrix[j][s-i];
  matrix[j][s-i] = tmp;
     }
 }
    }
}