<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 n, vector<vector<int>>&result, unordered_map<int,list<int>>&adj,vector<int>&visited,vector<int>&low,vector<int>&disc, int node,int parent,int &timer){
               visited[node]=true;
               disc[node]=low[node]=timer++;

               for(auto it:adj[node]){
                if(it==parent)
                continue;

                if(!visited[it]){
                  dfs(n,result,adj,visited,low,disc,it,node,timer);
                  low[node]=min(low[node],low[it]);

                  if(low[it]>disc[node]){
                    vector<int>ans;
                    ans.push_back(node);
                    ans.push_back(it);
                    result.push_back(ans);
                  }
                }
                else{
                     low[node]=min(low[node],disc[it]);
                }
               }
    }


    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        unordered_map<int,list<int>>adj;

        for(int i=0;i<connections.size();i++){
            int u=connections[i][0];
            int v=connections[i][1];
         
            adj[u].push_back(v);
            adj[v].push_back(u);

        }
        int timer=0;
        int parent=-1;
        vector<int>visited(n,0);
        vector<int>low(n,-1);
        vector<int>disc(n,-1);
        
        vector<vector<int>>result;
        for(int i=0;i<n;i++){
           dfs(n,result,adj,visited,low,disc,i,parent,timer);
        }

        return result;
    }
};
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