54. 螺旋矩阵

给定一个包含 m x n 个元素的矩阵（m 行, n 列），请按照顺时针螺旋顺序，返回矩阵中的所有元素。

示例 1:

输入:
[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]
输出: [1,2,3,6,9,8,7,4,5]
示例 2:

输入:
[
[1, 2, 3, 4],
[5, 6, 7, 8],
[9,10,11,12]
]
输出: [1,2,3,4,8,12,11,10,9,5,6,7]

class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
        int m = matrix.length;
        List<Integer> res = new ArrayList<>();
        if(m == 0) return res;
        int n = matrix[0].length;
        if(n == 0) return res;
        int up = 0, down = m - 1, left = 0, right = n - 1;
        while (true) {
            for (int j = left; j <= right; ++j) res.add(matrix[up][j]);
            if (++up > down) break;
            for (int i = up; i <= down; ++i) res.add(matrix[i][right]);
            if (--right < left) break;
            for (int j = right; j >= left; --j) res.add(matrix[down][j]);
            if (--down < up) break;
            for (int i = down; i >= up; --i) res.add(matrix[i][left]);
            if (++left > right) break;
        }
        return res;
    }
}