Spiral Matrix

Given an `m x n` `matrix`, return all elements of the `matrix` in spiral order. **Example 1:** Input: matrix = [[1,2,3],[4,5,6],[7,8,9]] Output: [1, 2, 3, 6, 9, 8, 7, 4, 5] **Example 2:** 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] **Example 3:** Input: matrix = [[1]] Output: [1] **Hint:** Use four boundary pointers (top, bottom, left, right) and shrink them as you traverse each layer of the spiral.