<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 {
    void dfs (std::vector<std::vector<int>>& graph, std::vector<bool>& visited, int src, int& count) {
        visited[src] = true;
        for (int i : graph[src]) {
            if (!visited[i]) {
                std::cout << i << ' ';
                dfs(graph, visited, i, count);
                ++count;                    
            }
        }
    }
public:
    long long countPairs(int n, vector<vector<int>>& edges) {
        std::vector<std::vector<int>> graph(n);
        for (const auto& v : edges) {
            graph[v[0]].push_back(v[1]);
            graph[v[1]].push_back(v[0]);
        }

        std::vector<long long>res{};
        std::vector<bool> visited(n, false);
        for (int i = 0; i < n; ++i) {
            if (!visited[i]) {
                int count = 1;
                dfs(graph, visited, i, count);
                res.push_back(count);
            }
        }
        long long ans = 0;
        for (int i = 0; i < res.size() - 1; i++) {
            ans = ans + (res[i] * res[i + 1]);
            res[i + 1] = res[i] + res[i + 1];
        }
        return ans;
    }
};
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