<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) {

        int rows = mat.size();
        int columns = mat[0].size();

        vector<vector<int>> visited_matrix(rows, vector<int>(columns, 0));

        vector<pair<int,int>> director = {{-1, +1}, {+1, -1} }; //NE is -1 row, +1 columns, SW is +1 row, -1 columns

        int direction = 0;
        int r = 0;
        int c = 0;

        vector<int> final_result;

        for (int i = 0; i < rows*columns; i++){
            final_result.push_back(mat[r][c]);

            int next_r = r + director[direction].first;
            int next_c = c + director[direction].second;

            if (next_r < 0 || next_r >= rows || next_c < 0 || next_c >= columns){
                
                if (direction == 0){ //if currently moving NE
                   if (c + 1 >= columns){
                        next_r = r + 1;
                        next_c = c;
                   } else{
                    next_r = r;
                    next_c = c+1;
                   }
                }
                else if (direction == 1){ //if currently moving SW
                   if (r + 1 >= rows){
                    next_c = c+1;
                    next_r = r;
                   } else{
                    next_r = r+1;
                    next_c = c;
                   }
                }

            direction = (direction + 1) % 2;
            
                

            }
            r = next_r;
            c = next_c;
        }
        return final_result;
    }
};
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