<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]:
        # up dir : row -1, col + 1
        # down dir: row + 1, col -1
        # if you go up dir and no more, then (row, col + 1) or (row + 1, col)
        # if you go down dir and no more, then (row +1, col) or (row, col + 1)

        res = []
        rows = len(mat)
        cols = len(mat[0])
        print(rows * cols)
        up_dir = True
        r, c = 0, 0
        while len(res) < rows * cols:
            if up_dir:
                while r in range(rows) and c in range(cols):
                    res.append(mat[r][c])
                    r -= 1
                    c += 1
                if c not in range(cols):
                    c -= 1
                    r += 2
                elif r not in range(rows):
                    r += 1
                up_dir = False
            else:
                while r in range(rows) and c in range(cols):
                    res.append(mat[r][c])
                    r += 1
                    c -= 1
                if r not in range(rows):
                    r -= 1
                    c += 2
                elif c not in range(cols):
                    c += 1
                up_dir = True
        
        return res
19500