<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>

bool init = []() {
   ios_base::sync_with_stdio(false);
   cin.tie(nullptr);
   cout.tie(nullptr);
   return true;
}();

class Solution {
public:
    vector<int> findDiagonalOrder(vector<vector<int>>& mat) {
        std::vector<int> ret;
        if (mat.empty())
            return ret;

        const int NumRows = mat.size();
        const int NumCols = mat[0].size();
        ret.reserve(NumRows*NumCols);

        std::vector<std::vector<int>> tmp(NumRows + NumCols, std::vector<int>{});
        for (int r=0; r < NumRows; ++r)
        {
            for (int c=0; c < NumCols; ++c)
            {
                assert(r+c < tmp.size());
                tmp[r+c].push_back(mat[r][c]);
            }
        }

        // for (int r=0; r < tmp.size(); ++r)
        //     std::cout << printVector(tmp[r]) << '\n';
        for (int i=0; i < tmp.size(); ++i)
        {
            if (i & 0x01)
            {
                for (int j=0; j < tmp[i].size(); ++j)
                    ret.push_back(tmp[i][j]);
            }
            else
            {
                for (int j=tmp[i].size()-1; j >= 0; --j)
                    ret.push_back(tmp[i][j]);
            }
        }
        return ret;
    }
};
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