<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;
    void dfs(vector<int>adj[], int *visited,int *lowest, vector<vector<int>>&ans,int parent,int node)
    {
        visited[node]=lowest[node]=time;
        time++;
        for(auto i:adj[node])
        {
            if(i==parent)continue;
            if(visited[i]==0)
            {
                dfs(adj,visited,lowest,ans,node,i);
                lowest[node]=min(lowest[i],lowest[node]);
                if(lowest[i]>visited[node])
                    ans.push_back({node,i});
            }
            else
            {
                lowest[node]=min(lowest[i],lowest[node]);
            }
        }
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
    vector<int>adj[n];
    time=1;
    for(auto i:connections)
    {
        adj[i[0]].push_back(i[1]);
        adj[i[1]].push_back(i[0]);
    }
    int visited[n];
    int lowest[n];
    for(int i=0;i<n;i++)
    {
        visited[i]=lowest[i]=0;
    }
    vector<vector<int>>ans;
    dfs(adj,visited,lowest,ans,-1,0);
    return ans;
    }
};
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