<p>You are given an integer <code>n</code>. There is an <strong>undirected</strong> graph with <code>n</code> nodes, numbered from <code>0</code> to <code>n - 1</code>. You are given a 2D integer array <code>edges</code> where <code>edges[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> denotes that there exists an <strong>undirected</strong> edge connecting nodes <code>a<sub>i</sub></code> and <code>b<sub>i</sub></code>.</p>

<p>Return <em>the <strong>number of pairs</strong> of different nodes that are <strong>unreachable</strong> from each other</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/05/05/tc-3.png" style="width: 267px; height: 169px;" />
<pre>
<strong>Input:</strong> n = 3, edges = [[0,1],[0,2],[1,2]]
<strong>Output:</strong> 0
<strong>Explanation:</strong> There are no pairs of nodes that are unreachable from each other. Therefore, we return 0.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/05/05/tc-2.png" style="width: 295px; height: 269px;" />
<pre>
<strong>Input:</strong> n = 7, edges = [[0,2],[0,5],[2,4],[1,6],[5,4]]
<strong>Output:</strong> 14
<strong>Explanation:</strong> There are 14 pairs of nodes that are unreachable from each other:
[[0,1],[0,3],[0,6],[1,2],[1,3],[1,4],[1,5],[2,3],[2,6],[3,4],[3,5],[3,6],[4,6],[5,6]].
Therefore, we return 14.
</pre>

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

<ul>
	<li><code>1 &lt;= n &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= edges.length &lt;= 2 * 10<sup>5</sup></code></li>
	<li><code>edges[i].length == 2</code></li>
	<li><code>0 &lt;= a<sub>i</sub>, b<sub>i</sub> &lt; n</code></li>
	<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>
	<li>There are no repeated edges.</li>
</ul>

//Burak Emre Polat
class Solution {
public:
    vector<vector<int>> adjacency;
    void maker(vector<vector<int>> &edges, int n) {
        unordered_map<int, vector<int>> um;
        for (auto x: edges) {
            um[x[0]].push_back(x[1]);
            um[x[1]].push_back(x[0]);
        }
        for (int i = 0; i < n; i++) {
            adjacency.push_back(um[i]);
        }
    }
    long long countPairs(int l, vector<vector<int>>& edges) {
        long long n = l;
        unordered_map<int, bool> visited;
        maker(edges, n);
        vector<int> groups;
        queue<int> q;
        for (int i = 0; i<n; i++) {
            if(!visited[i]) {
                long long size = 1;
                visited[i] = true;
                q.push(i);
                while (!q.empty()) {
                    int curr = q.front();
                    for(auto x: adjacency[curr]) {
                        if (!visited[x]) {
                            q.push(x);
                            visited[x] = true;
                            size++;
                        }
                    }
                    q.pop();
                }
                groups.push_back(size);
            }
        }
        long long res = 0;
        for (auto x: groups) {
            res += x*(n-x);
        }
        return res / 2;
    }
};
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