<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) {
        std::vector<std::vector<int>> diagonals{{}};

        for (int i = 0; i < mat.size()-1; i++) {
            int x = i;
            int y = 0;
            while (x >= 0 && y < mat[0].size()) {
                // printf("%d\n", mat[x][y]);
                diagonals[diagonals.size()-1].push_back(mat[x][y]);
                x--;
                y++;
            }
            diagonals.push_back({});
        }

        for (int j = 0; j < mat[0].size(); j++) {
            int x = mat.size()-1;
            int y = j;
            while (x >= 0 && y < mat[0].size()) {
                // printf("%d\n", mat[x][y]);
                diagonals[diagonals.size()-1].push_back(mat[x][y]);
                x--;
                y++;
            }
            diagonals.push_back({});
        }

        std::vector<int> result(mat.size() * mat[0].size());
        int idx = 0;
        for (int i = 0; i < diagonals.size(); i++) {
            if (i%2 == 1) {
                for (int j = diagonals[i].size() - 1; j >= 0; j--) {
                    result[idx] = diagonals[i][j];
                    idx++;
                }
            }
            else {
                for (int j = 0; j < diagonals[i].size(); j++) {
                    result[idx] = diagonals[i][j];
                    idx++;
                }
            }

        }

        return result;
    }
};
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