<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 node,vector<int> &vis,vector<int> &start,vector<int> &finish,vector<int> adj[],int &curr,int parent,vector<vector<int>> &ans)
    {
        vis[node]=1;
        curr++;
        start[node]=curr;
        finish[node]=curr;
        for(auto i:adj[node])
        {
            if(vis[i]==0)
            {
                dfs(i,vis,start,finish,adj,curr,node,ans);
                if(finish[i]>start[node])
                {
                   ans.push_back({node,i});
                }
            }
        }
        for(auto i:adj[node])
        {
            if(i!=parent)
            {
                finish[node]=min(finish[node],finish[i]);
            }
        }
        return;

    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& conn) {
        vector<int> adj[n];
        for(int i=0;i<conn.size();i++)
        {
            int u=conn[i][0];
            int v=conn[i][1];
            adj[u].push_back(v);
            adj[v].push_back(u);

        }
        vector<int> vis(n,0);
        vector<int> start(n,0);
        vector<int> finish(n,0);
        vector<vector<int>> ans;
        int curr=0;
        dfs(0,vis,start,finish,adj,curr,-1,ans);
        return ans;
    }
};
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