<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 n = mat.size(), m = mat[0].size();

        int i = 0, j = 1;
        bool flag = true;
        vector<int>ans(n*m);
        int iter = 0;
        int tot = 0;
        while(tot < m + n - 1){
            if(flag){
                if(i < n){
                    int ii = i;
                    int jj = 0;
                    while(ii >=0  && jj < m){
                        ans[iter++] = mat[ii][jj];
                        ii--;
                        jj++;
                    }
                    i++;
                }
                else{
                    int ii = n - 1;
                    int jj = j;
                    while(ii >=0  && jj < m){
                        ans[iter++] = mat[ii][jj];
                        ii--;
                        jj++;
                    }
                    j++;
                }
            }
            else{
                if(i < n){
                    int ii = i;
                    int jj = 0;
                    vector<int>t;
                    while(ii >=0  && jj < m){
                        t.push_back(mat[ii][jj]);
                        ii--;
                        jj++;
                    }
                    reverse(t.begin(), t.end());
                    for(int it=0; it<t.size(); it++){
                        ans[iter++] = t[it];
                    }
                    i++;
                }
                else{
                    int ii = n - 1;
                    int jj = j;
                    vector<int>t;
                    while(ii >=0  && jj < m){
                        t.push_back(mat[ii][jj]);
                        ii--;
                        jj++;
                    }
                    reverse(t.begin(), t.end());
                    for(int it=0; it<t.size(); it++){
                        ans[iter++] = t[it];
                    }
                    j++;
                }
            }
            flag = !flag;
            tot++;
        }
        return ans;
    }
};
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