Given an `m x n` `matrix`, return _all elements of the_ `matrix` _in spiral order_.

Example 1:

![](https://assets.leetcode.com/uploads/2020/11/13/spiral1.jpg)
    
    Input: matrix = [[1,2,3],[4,5,6],[7,8,9]] Output: [1,2,3,6,9,8,7,4,5] 

Example 2:

![](https://assets.leetcode.com/uploads/2020/11/13/spiral.jpg)
    
    Input: matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]] Output: [1,2,3,4,8,12,11,10,9,5,6,7] 

Constraints:

* `m == matrix.length`
* `n == matrix[i].length`
* `1 <= m, n <= 10`
* `-100 <= matrix[i][j] <= 100`