<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 u,vector<int> adj[],vector<int> &vis,vector<int> &ans,vector<int> &low,vector<int> &disc,int parent,vector<vector<int>> &res){

        low[u]=disc[u]=timer++;
        vis[u]=1;

        for(auto v : adj[u]){
            if(v==parent) continue;
            if(!vis[v]){
                dfs(v,adj,vis,ans,low,disc,u,res);
                low[u]=min(low[u],low[v]);

                if(low[v]>disc[u]){
                    vector<int> vec={u,v};
                    res.push_back(vec);
                }

            }else{
                low[u]=min(low[u],disc[v]);
            }
        }

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

        vector<vector<int>> res;
        vector<int> vis(n,0),ans(n,0),low(n),disc(n);
        for(int i=0;i<n;i++){
            if(!vis[i]){
                dfs(i,adj,vis,ans,low,disc,-1,res);
            }
        }
        return res;
    }
};
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