<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:
  
vector<int>adj[100005];
int timein[100005];
int low[100005];
int timer=0;
int vis[100005];

vector<vector<int>>ans;

void dfs(int node,int par)
{
    if(vis[node])return;
    vis[node] = 1;
    timein[node] = low [node] = timer++;
    for(auto it : adj[node])
    {
        if(it == par)continue;
        if(vis[it])
        {
            low[node] = min(low[node],timein[it]);
        }
        else
        {
            dfs(it,node);
            if(low[it]>timein[node])
            {
                ans.push_back({node,it});
            }
            low[node] = min(low[node],low[it]);  
        }
    }
}

vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) 
{
    for(auto it : connections)
    {
        adj[it[0]].push_back(it[1]);
        adj[it[1]].push_back(it[0]);
    }
    for(int i=0;i<n;i++)
    {
        if(vis[i]==0)
        {
            dfs(i,-1);
        }
    }
    return ans;
}
};
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