<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) {
        vector<int> res;
        if (mat.empty() || mat[0].empty()) {
            return res;
        }
        int m = mat.size();
        int n = mat[0].size();
        bool goingUp = true;
        int i = 0;
        int j = 0;
        unordered_set<int> seen;
        queue<int> q;
        q.push(i * n + j);

        vector<vector<int>> directions = {{-1, 1}, {1, -1}};
        while (!q.empty()) {
            int sz = q.size();
            for (int k = 0; k < sz; ++k) {
                i = q.front() / n;
                j = q.front() % n;
                q.pop();
                res.push_back(mat[i][j]);
                if (i == m - 1 && j == n - 1) {
                    break;
                }
                if (goingUp) {
                    if (j == n - 1) {
                        i = i + 1;
                        goingUp = false;
                    } else if (i == 0) {
                        j = j + 1;
                        goingUp = false;
                    } else {
                        i = i - 1;
                        j = j + 1;
                    }
                } else {
                    if (i == m - 1) {
                        j = j + 1;
                        goingUp = true;
                    } else if (j == 0) {
                        i = i + 1;
                        goingUp = true;
                    } else {
                        i = i + 1;
                        j = j - 1;
                    }
                }
                q.push(i * n + j);
            }
        }
        return res;
        
    }
};
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