Diagonal Traverse
<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>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0400-0499/0498.Diagonal%20Traverse/images/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 class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> mat = [[1,2],[3,4]]
<strong>Output:</strong> [1,2,3,4]
</pre>

<p>&nbsp;</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 &lt;= m, n &lt;= 10<sup>4</sup></code></li>
	<li><code>1 &lt;= m * n &lt;= 10<sup>4</sup></code></li>
	<li><code>-10<sup>5</sup> &lt;= mat[i][j] &lt;= 10<sup>5</sup></code></li>
</ul>
Array; Matrix; Simulation
function findDiagonalOrder(mat: number[][]): number[] {
    const res = [];
    const m = mat.length;
    const n = mat[0].length;
    let i = 0;
    let j = 0;
    let mark = true;
    while (res.length !== n * m) {
        if (mark) {
            while (i >= 0 && j < n) {
                res.push(mat[i][j]);
                i--;
                j++;
            }
            if (j === n) {
                j--;
                i++;
            }
            i++;
        } else {
            while (i < m && j >= 0) {
                res.push(mat[i][j]);
                i++;
                j--;
            }
            if (i === m) {
                i--;
                j++;
            }
            j++;
        }
        mark = !mark;
    }
    return res;
}
