螺旋矩阵

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example 1:

Input:
[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]
Output: [1,2,3,6,9,8,7,4,5]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/spiral-matrix
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

建四个边界，up,down,left,right,逆时针方向游走，把遍历到的元素一次加到list里面，当每一行或每一列遍历完时，边界自增或自减。

public List<Integer> spiralOrder(int[][] matrix)
    {
        List<Integer> list = new ArrayList<Integer>();
        if(matrix == null || matrix.length == 0)
        {
            return list;
        }
        int up = 0;
        int d = matrix.length-1;
        int left = 0;
        int right = matrix[0].length-1;
        while(up<=d || left<=right)
        {
            //从左到右
            for(int i = left; i <= right; i++)
            {
                list.add(matrix[up][i]);
            }
            up++;
            if(up > d)
            {
                break;
            }
            //从上到下
            for(int i = up; i <= d; i++)
            {
                list.add(matrix[i][right]);
            }
            right--;
            if(left > right)
            {
                break;
            }
            //从右往左
            for(int i = right; i >= left; i--)
            {
                list.add(matrix[d][i]);
            }
            d--;
            if(up > d)
            {
                break;
            }
            //从下往上
            for(int i = d; i >= up; i--)
            {
                list.add(matrix[i][left]);
            }
            left++;
            if(left > right)
            {
                break;
            }
        }
        return list;
    }

运行效果如图：