<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 preparelist(unordered_map<int,list<int>>& m , vector<vector<int>>& connections){
        for(auto i:connections){
            int u=i[0];
            int v=i[1];
            m[u].push_back(v);
            m[v].push_back(u);
        }
    }
    void dfs(int node , int parent, unordered_map<int,list<int>>&m , vector<int>& visited , vector<int>& time , 
    vector<int>&ltime , vector<vector<int>>&ans , int timer){
           time[node]=ltime[node]=timer;
           visited[node]=1;
           timer++;
           for(auto it:m[node]){
               if(it==parent){
                continue;
               }
               if(visited[it]!=1){
                dfs(it,node,m,visited,time,ltime,ans,timer);
                ltime[node]=min(ltime[node],ltime[it]);
                if(ltime[it]>time[node]){
                ans.push_back({it,node});
               }
               }
               else{
                ltime[node]=min(ltime[node],ltime[it]);
               }   
           }
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
         unordered_map<int,list<int>>m;
         preparelist(m,connections);
         vector<int> visited(n, 0);
        vector<int> time(n, -1);
        vector<int> ltime(n, -1);
        vector<vector<int>> ans;
        int timer = 0;      
         dfs(0,-1,m,visited,time,ltime,ans,timer);
         return ans;
    }
};
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