<p>Given an <code>m x n</code> matrix <code>mat</code>, return <em>an array of all the elements of the array in a diagonal order</em>.</p> <p> </p> <p><strong>Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2021/04/10/diag1-grid.jpg" style="width: 334px; height: 334px;" /> <pre> <strong>Input:</strong> mat = [[1,2,3],[4,5,6],[7,8,9]] <strong>Output:</strong> [1,2,4,7,5,3,6,8,9] </pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> mat = [[1,2],[3,4]] <strong>Output:</strong> [1,2,3,4] </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>m == mat.length</code></li> <li><code>n == mat[i].length</code></li> <li><code>1 <= m, n <= 10<sup>4</sup></code></li> <li><code>1 <= m * n <= 10<sup>4</sup></code></li> <li><code>-10<sup>5</sup> <= mat[i][j] <= 10<sup>5</sup></code></li> </ul>