顺时针打印矩阵

public class clockWiseOrderPrint {
    public static void main(String[] args) {
        int[][] matrix = new int[][]
            {
                {1, 4, 7, 11, 15},
                {2, 5, 8, 12, 19},
                {3, 6, 9, 16, 22},
                {10, 13, 14, 17, 24},
                {18, 21, 23, 26, 30}
            };
        System.out.println(new clockWiseOrderPrint().printMatrix(matrix));
    }
    public List<Integer> printMatrix(int[][] matrix) {
        List<Integer> ret = new ArrayList<>();
        //r1：最上面第一行。r2：最下面一行。c1：最左边的一列。c2：最右边的一列。
        int r1 = 0, r2 = matrix.length - 1, c1 = 0, c2 = matrix[0].length - 1;
        while (r1 <= r2 && c1 <= c2) {
            // 上
            for (int i = c1; i <= c2; i++)
                ret.add(matrix[r1][i]);
            // 右
            for (int i = r1 + 1; i <= r2; i++)
                ret.add(matrix[i][c2]);
            if (r1 != r2)
                // 下
                for (int i = c2 - 1; i >= c1; i--)
                    ret.add(matrix[r2][i]);
            if (c1 != c2)
                // 左
                for (int i = r2 - 1; i > r1; i--)
                    ret.add(matrix[i][c1]);
            r1++; r2--; c1++; c2--;
        }
        return ret;
    }
}