<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::unordered_map<int, std::vector<int>> vec_map;
        int max_num = 0;
        for(int i = 0; i < mat.size(); i++) {
            for (int j = 0;  j < mat[i].size(); j++) {
                vec_map[i+j].push_back(mat[i][j]);
                max_num = std::max(max_num, i+j);
            }
        }

        std::vector<int> res;
        for (int i = 0; i <= max_num; i++) {
            const auto& cur = vec_map[i];
            if (i % 2 == 0) {
                for (int j = cur.size() - 1; j >= 0; j--) {
                    res.push_back(cur[j]);
                }
            } else {
                for (int j = 0; j < cur.size(); j++) {
                    res.push_back(cur[j]);
                }
            }
        }
        return res;
    }
};
29108