<p>Given an <code>m x n</code> <code>matrix</code>, return <em>all elements of the</em> <code>matrix</code> <em>in spiral order</em>.</p> <p> </p> <p><strong class="example">Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2020/11/13/spiral1.jpg" style="width: 242px; height: 242px;" /> <pre> <strong>Input:</strong> matrix = [[1,2,3],[4,5,6],[7,8,9]] <strong>Output:</strong> [1,2,3,6,9,8,7,4,5] </pre> <p><strong class="example">Example 2:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2020/11/13/spiral.jpg" style="width: 322px; height: 242px;" /> <pre> <strong>Input:</strong> matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]] <strong>Output:</strong> [1,2,3,4,8,12,11,10,9,5,6,7] </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>m == matrix.length</code></li> <li><code>n == matrix[i].length</code></li> <li><code>1 <= m, n <= 10</code></li> <li><code>-100 <= matrix[i][j] <= 100</code></li> </ul>