Rotate Image [LeetCode]

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:

    void rotate(vector<vector<int> > &matrix) {
        int n = matrix.size();
        for(int i = 0; i < n / 2; i ++) {
            for(int j = i; j < n - 1 - i; 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;                    
            }
        }
    }