public ArrayList<Integer> printMatrix(int[][] matrix) {
        if (matrix == null) return null;

        int col = matrix[0].length;
        int row = matrix.length;
        ArrayList<Integer> res = new ArrayList<>();

        int tmp = Math.min(col, row);
        int circle = ((tmp & 1) == 1) ? (tmp / 2 + 1) : (tmp / 2);

        int count = 0;
        int all = col * row;
        int stX = 0;
        int stY = 0;
        int endX = col - 1;
        int endY = row - 1;

        for (int cir = 1; cir <= circle; cir++) {
//            up
            for (int i = stX; i <= endX; i++) {
                res.add(matrix[cir - 1][i]);
                count++;

            }
            if (count == all) break;
//            改变right方向的起始位置
            stY++;

//            right
            for (int i = stY; i <= endY; i++) {
                res.add(matrix[i][col - cir]);
                count++;
            }
            if (count == all) break;
//            改变down方向的起始位置
            endX--;

//            down
            for (int i = endX; i >= stX; i--) {
                res.add(matrix[row - cir][i]);
                count++;
            }
            if (count == all) break;
//            改变left的起始位置
            endY--;

//            left
            for (int i = endY; i >= stY; i--) {
                res.add(matrix[i][cir - 1]);
                count++;
            }
            if (count == all) break;
//            改变up方向的起始位置
            stX++;
        }
        return res;
    }