<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:
int ROWS;
int COLS;
    bool isInMat(int r, int c) {
        return r >= 0 && r < ROWS && c >= 0 && c < COLS;
    }
    vector<int> findDiagonalOrder(vector<vector<int>>& mat) {

        std::vector<int> output;

        ROWS = mat.size();
        COLS = mat[0].size();

        std::vector<std::pair<int, int>> positions;
        for (int c = 0; c < COLS; ++c) {
            positions.emplace_back(std::make_pair(0, c));
        }
        for (int r = 1; r < ROWS; ++r) {
            positions.emplace_back(std::make_pair(r, COLS - 1));
        }

        bool isReversed = true;
        std::vector<int> diag;
        for (const auto& pos : positions){
            int row = pos.first;
            int col = pos.second;
            diag.clear();
            while (isInMat(row, col)) {
                diag.push_back(mat[row][col]);
                row += 1;
                col -= 1;
            }

            if (isReversed){
                std::reverse(diag.begin(), diag.end());
            }
            isReversed = !isReversed;
            output.insert(output.end(), diag.begin(), diag.end());
        }

        return output;
    }
};
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