<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 time=0;
    vector<vector<int>> ans;
    void dfs(int start,int parent, vector<vector<int>> &adj,vector<bool> &vis,vector<int> &low,vector<int> &disc)
    {
        if(vis[start]) return;
       vis[start]=true;
       disc[start]=low[start]=++time;
       for(auto v:adj[start])
       {
        if(v==parent) continue;
        if(vis[v]){low[start]=min(low[start],disc[v]);}
        else 
        {
            dfs(v,start,adj,vis,low,disc);
            low[start]=min(low[start],low[v]);
            if(low[v]>disc[start]) ans.push_back({start,v});
        }
       }

    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<int> low(n,-1),disc(n,-1);
        vector<vector<int>> adj(n,vector<int>());
        for(auto v:connections)
        {
            adj[v[0]].push_back(v[1]);
            adj[v[1]].push_back(v[0]);
        }
        vector<bool> vis(n,false);
        dfs(0,-1,adj,vis,low,disc);
        return ans;
    }
};
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