<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(), n=mat[0].size();
        queue<pair<int,int>> q;
        vector<int> final;
        final.push_back(mat[0][0]);
        vector<vector<int>> visited(m,vector<int>(n,0));
        q.push({0,0});
        int flag=1;
        visited[0][0]=1;
        while(!q.empty()){
            int s=q.size();
            vector<int> ans;
            for(int i=0;i<s;i++){
                int r=q.front().first;
                int c=q.front().second;
                q.pop();
                if(c+1<n and !visited[r][c+1]){
                    ans.push_back(mat[r][c+1]);
                    visited[r][c+1]=1;
                    q.push({r,c+1});
                }
                if(r+1<m and !visited[r+1][c]){
                    ans.push_back(mat[r+1][c]);
                    visited[r+1][c]=1;
                    q.push({r+1,c});
                }
            }
            if(flag){
                for(int i=0;i<ans.size();i++) final.push_back(ans[i]);
            }
            else{
                for(int i=ans.size()-1;i>=0;i--) final.push_back(ans[i]);
            }
            flag=1-flag;
        }
        return final;        
    }
};
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