<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 createAdjList(vector<vector<int>>& connections,unordered_map<int,vector<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);
        }

    }

    void dfs(vector<int> &disc,vector<int> &low,int parent,int &timer,unordered_map<int,vector<int>> &adj,vector<vector<int>> &ans,int node){

        disc[node]=timer;
        low[node]=timer;
        timer++;

        for(auto nbr:adj[node]){
            if(nbr==parent){
                continue;
            }
            else if(disc[nbr]==-1){
                dfs(disc,low,node,timer,adj,ans,nbr);
                low[node]=min(low[node],low[nbr]);
                if(low[nbr]>disc[node]){
                    ans.push_back({node,nbr});
                }
            }
            else{
                low[node]=min(low[node],disc[nbr]);
            }
        }

    }

    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {

        vector<int> disc(n,-1);
        vector<int> low(n,-1);
        int parent=-1;
        int timer=0;
        unordered_map<int,vector<int>> adj;
        vector<vector<int>> ans;

        createAdjList(connections,adj);

        for(int i=0;i<n;i++){
            if(disc[i]==-1){
                dfs(disc,low,parent,timer,adj,ans,i);
            }
        }

        return ans;
        
    }
};
186756