<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]:
        direction = 1 #1 : Up, 0: Down
        row, col = 0, 0
        result = []

        def check_bounds(i, j):
            if i < 0 or j < 0:
                return False
            if i > len(mat) - 1 or j > len(mat[i]) - 1:
                return False
            return True

        while check_bounds(row, col):
            result.append(mat[row][col])
            if direction:
                row -= 1
                col += 1
            else:
                row += 1
                col -= 1
            if not check_bounds(row, col):
                if direction == 1:
                    col -= 1
                    row += 1
                    if check_bounds(row, col + 1):
                        row, col = row, col+1
                    else:
                        row, col = row+1, col
                else:
                    row -= 1
                    col += 1
                    if check_bounds(row + 1, col):
                        row, col = row + 1, col
                    else:
                        row, col = row, col + 1
                direction = 1 - direction
        return result
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