### 54. 螺旋矩阵
【题目描述】给你一个 m 行 n 列的矩阵 matrix ,请按照 顺时针螺旋顺序 ,返回矩阵中的所有元素。
示例 1:
![img](https://assets.leetcode.com/uploads/2020/11/13/spiral1.jpg)
```
输入:matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出:[1,2,3,6,9,8,7,4,5]
```
示例 2:
![img](https://assets.leetcode.com/uploads/2020/11/13/spiral.jpg)
```
输入:matrix = [[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]
```
【解题思路】可以发现,遍历的顺序永远都是 **右->下->左->上**。维护四个边界,每走完一个顺序就收缩相应的边界。强推大家看看这篇题解:[【动画模拟】一下就能搞懂,很简单](https://leetcode-cn.com/problems/spiral-matrix/solution/dong-hua-mo