<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://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 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>

class Solution:
    def findDiagonalOrder(self, mat: List[List[int]]) -> List[int]:
        n_rows, n_cols = len(mat), len(mat[0])
        diagonal_sum_vals = dict()
        for row_id in range(n_rows):
            for col_id in range(n_cols):
                diagonal_sum_vals[row_id + col_id] = diagonal_sum_vals.get(row_id + col_id, []) + [mat[row_id][col_id]]
        result = list()
        for s in range(min(diagonal_sum_vals.keys()), max(diagonal_sum_vals.keys()) + 1):
            if s % 2 == 0:
                while len(diagonal_sum_vals[s]) > 0:
                    result.append(diagonal_sum_vals[s].pop(-1))
            else:
                while len(diagonal_sum_vals[s]) > 0:
                    result.append(diagonal_sum_vals[s].pop(0))
        return result
20000