class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
		List<Integer> result = new ArrayList<>();
        if (matrix.length == 0)
            return result;
        //设定上下左右边界
        int up = 0;
        int down = matrix.length-1;
        int left = 0;
        int right = matrix[0].length-1;

        while (true) {
            //向右移动到最右
            for (int i = left; i <= right; i++) {
                result.add(matrix[up][i]);
            }
            up++;
            if (up > down) {
                break;
            }
            //向下移动
            for (int i = up; i <= down; i++) {
                result.add(matrix[i][right]);
            }
            right--;
            if (right < left) {
                break;
            }
            //向左移动
            for (int i = right; i >= left; i--) {
                result.add(matrix[down][i]);
            }
            down--;
            if (down < up) {
                break;
            }
            //向上移动
            for (int i = down; i >= up; i--) {
                result.add(matrix[i][left]);
            }
            left++;
            if (left > right) {
                break;
            }
        }

        return result;
    }
}