<p>Given a 2D integer array <code>nums</code>, return <em>all elements of </em><code>nums</code><em> in diagonal order as shown in the below images</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/04/08/sample_1_1784.png" style="width: 158px; height: 143px;" />
<pre>
<strong>Input:</strong> nums = [[1,2,3],[4,5,6],[7,8,9]]
<strong>Output:</strong> [1,4,2,7,5,3,8,6,9]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/04/08/sample_2_1784.png" style="width: 230px; height: 177px;" />
<pre>
<strong>Input:</strong> nums = [[1,2,3,4,5],[6,7],[8],[9,10,11],[12,13,14,15,16]]
<strong>Output:</strong> [1,6,2,8,7,3,9,4,12,10,5,13,11,14,15,16]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= nums[i].length &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= sum(nums[i].length) &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= nums[i][j] &lt;= 10<sup>5</sup></code></li>
</ul>

class Solution {
public:
    vector<int> findDiagonalOrder(vector<vector<int>>& nums) {
        queue<pair<int, int>> q;
        vector<int> out;
        set<pair<int, int>> visited;  // Track visited pairs

        q.push({0, 0});
        visited.insert({0, 0});

        while (!q.empty()) {
            auto [x, y] = q.front(); 
            q.pop();
            if (x < nums.size() && y < nums[x].size()) {
                out.push_back(nums[x][y]);
            }
            if (x + 1 < nums.size() && y < nums[x + 1].size() && visited.find({x + 1, y}) == visited.end()) {
                q.push({x + 1, y});
                visited.insert({x + 1, y});
            }
            if (y + 1 < nums[x].size() && visited.find({x, y + 1}) == visited.end()) {
                q.push({x, y + 1});
                visited.insert({x, y + 1});
            }
        }
        
        return out;
    }
};

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