题解 | #顺时针打印矩阵#

    public ArrayList<Integer> printMatrix(int [][] matrix) {

        if (matrix == null || matrix.length == 0){
            return new ArrayList<>();
        }

        int up = 0;
        int right = matrix[0].length - 1;
        int down = matrix.length - 1;
        int left = 0;

        ArrayList<Integer> res = new ArrayList<>();

        while (left <= right && up <= down){

            for (int i = left; i <= right; i++) {

                res.add(matrix[up][i]);
            }

            for (int i = up + 1; i <= down; i++) {

                res.add(matrix[i][right]);
            }

            for (int i = right - 1; i >= left && left < right && down > up; i--) {

                res.add(matrix[down][i]);
            }

            for (int i = down - 1; i > up && down > up && left < right; i--) {

                res.add(matrix[i][left]);
            }

            up ++;
            right --;
            down --;
            left ++;
        }


        return res;
    }