<p>There are <code>n</code> servers numbered from <code>0</code> to <code>n - 1</code> connected by undirected server-to-server <code>connections</code> forming a network where <code>connections[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> represents a connection between servers <code>a<sub>i</sub></code> and <code>b<sub>i</sub></code>. Any server can reach other servers directly or indirectly through the network.</p>

<p>A <em>critical connection</em> is a connection that, if removed, will make some servers unable to reach some other server.</p>

<p>Return all critical connections in the network in any order.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/09/03/1537_ex1_2.png" style="width: 198px; height: 248px;" />
<pre>
<strong>Input:</strong> n = 4, connections = [[0,1],[1,2],[2,0],[1,3]]
<strong>Output:</strong> [[1,3]]
<strong>Explanation:</strong> [[3,1]] is also accepted.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 2, connections = [[0,1]]
<strong>Output:</strong> [[0,1]]
</pre>

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

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

class Solution {
public:
    void dfs(int i, int p, vector<int>& v, int v1[], int v2[], int& cnt, vector<int>adj[], vector<vector<int>>& ans){
        v[i]=1;
        v1[i]=cnt;
        v2[i]=cnt;
        cnt++;
        for(auto it: adj[i]){
            if(it==p){continue;}
            if(v[it]){v1[i]=min(v1[i], v1[it]);}
            else{
                dfs(it, i, v, v1, v2, cnt, adj, ans);
                v1[i]=min(v1[i], v1[it]);
                if(v1[it]>v2[i]){
                    ans.push_back({i, it});
                }
            }
        }
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<int>adj[n];
        for(int i=0; i<connections.size(); i++){
            adj[connections[i][0]].push_back(connections[i][1]);
            adj[connections[i][1]].push_back(connections[i][0]);
        }
        int cnt=1;
        vector<int>v(n, 0);
        int v1[n], v2[n];
        vector<vector<int>>ans;
        dfs(0, -1, v, v1, v2, cnt, adj, ans);
        return ans;
    }
};
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