坐标变换

    public int[][] rotateMatrix(int[][] mat, int n) {
        // write code here

        if (mat == null){
            return null;
        }

        if (n <= 1){
            return mat;
        }

        int[][] res = new int[n][n];

        for (int i = 0; i < n; i++) {

            for (int j = 0; j < n; j++) {

                res[j][n-i-1] = mat[i][j];


            }
        }

        return res;
    }