<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:
    int timer=0;
    void dfs(int parent,int ver,vector<int>&tim,vector<int>&low,set<vector<int>>&check,vector<int>&vis,vector<vector<int>>&v){
        vis[ver]=1;
        tim[ver]=++timer;
        low[ver]=timer;
        for(auto i:v[ver]){
            if(i==parent)continue;
            if(!vis[i]){
                dfs(ver,i,tim,low,check,vis,v);
                low[ver]=min(low[ver],low[i]);
            }
            else
                low[ver]=min(low[ver],tim[i]);
        }
        if(parent!=-1){
            if(low[ver]>tim[parent]){
                check.insert({min(ver,parent),max(ver,parent)});
            }
        }
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
     timer=0;
     vector<vector<int>>v(n);
     int m=connections.size();
     for(int i=0;i<m;i++){
        int x=connections[i][0];
        int y=connections[i][1];
        v[x].push_back(y);
        v[y].push_back(x);
     }   
     vector<int>vis(n,0);
     set<vector<int>>check;
     vector<int>low(n,0);
     vector<int>tim(n,0);
     vector<vector<int>>answer;
     for(int i=0;i<n;i++){
        if(vis[i]==0){
            timer=0;
            dfs(-1,i,tim,low,check,vis,v);
        }
     }
     for(auto i:check)answer.push_back(i);
     return answer;
    }
};
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