<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=1;
   void dfs(int node, int parent,vector<int>&vis,vector<vector<int>>&adj,vector<int>&low,vector<int>&tn,vector<vector<int>>&ans){
           vis[node]=1;
           tn[node]=timer;
           low[node]=timer;
           timer++;
           for(int adjnode:adj[node]){
              if(adjnode==parent)
              continue;
              if(vis[adjnode]==0){
                dfs(adjnode,node,vis,adj,low,tn,ans);
             
                if(low[adjnode]>tn[node]){
                    ans.push_back({node,adjnode});
                }
                 low[node]=min(low[adjnode],low[node]);
              }
              else {
                  low[node]=min(low[adjnode],low[node]);
              }
           }
   }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
                int p=connections.size();
                vector<vector<int>>adj(n);
                for(int i=0; i<p; i++){
                    adj[connections[i][0]].push_back(connections[i][1]);
                     adj[connections[i][1]].push_back(connections[i][0]);

                }
                vector<int>low(n,0),tn(n,0),vis(n,0);
                vector<vector<int>>ans;
                dfs(0,-1,vis,adj,low,tn,ans);
                return ans;

    }
};
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