<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 {
public:
    vector<int> findDiagonalOrder(vector<vector<int>>& mat) {
        vector<int> nums;
        int row = 0;
        int col = 0;
       
        bool up = true;
        
        while (true) {
            nums.push_back(mat[row][col]);
            
            if (row == mat.size()-1 && col == mat[0].size()-1) {
                break;
            }
            
            if (up) {
                // ascending diagonal 
                // check if we hit border
                if (col == mat[0].size()-1) {
                    row++;
                    up = false;
                    continue;
                } else if (row == 0) {
                    col++;
                    up = false;
                    continue;
                }
                
                // update the indices
                row--;
                col++;
                
            } else {
                // descending diagonal
                // check if we hit border
                if (row == mat.size()-1) {
                    col++;
                    up = true;
                    continue;
                } else if (col == 0) {
                    row++;
                    up = true;
                    continue;
                }
                
                // update the indices
                row++;
                col--;
                
            }
            
        }
        
        return nums;
    }
};
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