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
func findDiagonalOrder(mat [][]int) []int {
	m, n := len(mat), len(mat[0])
	var ans []int
	for k := 0; k < m+n-1; k++ {
		var t []int
		i, j := k-n+1, n-1
		if k < n {
			i, j = 0, k
		}
		for i < m && j >= 0 {
			t = append(t, mat[i][j])
			i++
			j--
		}
		if k%2 == 0 {
			p, q := 0, len(t)-1
			for p < q {
				t[p], t[q] = t[q], t[p]
				p++
				q--
			}
		}
		for _, v := range t {
			ans = append(ans, v)
		}
	}
	return ans
}