<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 m = mat.size();
        int n = mat[0].size();
        queue<vector<int>> m_que;
        m_que.push({0, 0});
        vector<int> result;
        bool up = true;
        while (!m_que.empty()) {
            int size = m_que.size();
            vector<int> level;
            for (int i = 0; i < size; i++) {
                int cur_i = m_que.front()[0];
                int cur_j = m_que.front()[1];
                level.push_back(mat[cur_i][cur_j]);
                m_que.pop();
                if (cur_j == 0 && cur_i < m - 1) {
                    m_que.push({{cur_i + 1, cur_j}});
                }
                if (cur_j < n - 1) {
                    m_que.push({cur_i, cur_j + 1});
                }
            }
            if (up) {
                for (int i = 0; i < level.size(); i++) {
                    result.push_back(level[i]);
                }
            } else {
                for (int i = level.size() - 1; i >= 0; i--) {
                    result.push_back(level[i]);
                }
            }
            up = !up;
        }
        return result;
    }
};
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