<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>

class Solution {
public:
    long long bfs( vector<vector<long long>> &adj, vector<long long>&visited, int n, int i){
        queue<int>qu;
        qu.push(i);
        long long cnt=0;
        while(!qu.empty()){
            long long ele = qu.front();
            qu.pop();
            if(!visited[ele]){
                            cnt++;
            }
            visited[ele] = 1;
            for(int x=0;x<adj[ele].size();++x)
            {
                if(!visited[adj[ele][x]]){
                    qu.push(adj[ele][x]);
                }
            }
        }
        return cnt;
    }
    long long countPairs(int n, vector<vector<int>>& edges) {
        vector<vector<long long>> adj(n);
        vector<long long>visited(n);
        for(int i=0;i<edges.size();++i)
        {
            adj[edges[i][0]].push_back(edges[i][1]);
            adj[edges[i][1]].push_back(edges[i][0]);
        }
        // for(int i=0;i<adj.size();++i){
        //     cout<<i<<"-->";
        //     for(int j=0;j<adj[i].size();++j)
        //     {
        //         cout<<adj[i][j]<< " ";
        //     }
        //     cout<<endl;
        // }
        vector<long long> aux;
        long long visCount=n;
        long long res=0;
        for(int i=0;i<n;++i)
        {
            if(!visited[i]){
                int loadBfs = bfs(adj,visited,n,i);
                // cout<<loadBfs<<endl;
                res+=loadBfs*(visCount-loadBfs);
                visCount-=loadBfs;
            }
        }
        
        return res;
    }
};
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