<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> vec;

    bool isValid(int x, int y, vector<vector<int>>& mat){
        return (x >= 0 && x < mat.size() && y >= 0 && y < mat[0].size());
    }
    void dfs(int x, int y, vector<vector<int>>& mat, bool dir){
        vec.push_back(mat[x][y]);
        if(isValid(x - 1, y + 1, mat) && dir == 1){
            dfs(x - 1, y + 1, mat, 1);
        } else if(isValid(x + 1, y - 1, mat) && dir == 0){
            dfs(x + 1, y - 1, mat, 0);
        } else if (isValid(x, y + 1, mat) && dir == 1){
            dfs(x, y + 1, mat, 0);
        } else if (isValid(x + 1, y, mat) && dir == 0){
            dfs(x + 1, y, mat, 1);
        } else if (y == mat[0].size() -1 && isValid(x + 1, y, mat) && dir == 1){
            dfs(x + 1, y, mat, 0);
        } else if (x == mat.size()-1 && isValid(x, y + 1, mat) && dir == 0){
            dfs(x , y + 1, mat, 1);
        }
    }
    vector<int> findDiagonalOrder(vector<vector<int>>& mat) {
        vector<vector<bool>> visited(mat.size(), vector<bool>(mat[0].size(), 0));
        dfs(0, 0, mat, 1);
        return vec;
    }
};
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