<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 count;
    vector<list<int>> graph;
    void add_edge(vector<vector<int>>& connections){
        for(int i=0;i<connections.size();i++){
            graph[connections[i][0]].push_back(connections[i][1]);
            graph[connections[i][1]].push_back(connections[i][0]);
        }
    }
    void dfs(int node,int parent,vector<int> &disc,vector<int> &low,vector<vector<int>> &bridges,unordered_set<int> &vis){
        disc[node]=low[node]=count;
        vis.insert(node);
        for(auto neighbour:graph[node]){
            if(neighbour==parent) continue;
            else if(vis.count(neighbour)>0){
                low[node]=min(low[node],low[neighbour]);
            }
            else{
                count++;
                dfs(neighbour,node,disc,low,bridges,vis);
                if(low[neighbour]>disc[node]){
                    //Bridge found
                    bridges.push_back({node,neighbour});
                }
                low[node]=min(low[node],low[neighbour]);
            }

        }
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        count=0;
        graph.clear();
        graph.resize(n);
        add_edge(connections);
        vector<int> disc(n);
        vector<int> low(n);
        vector<vector<int>> bridges;
        unordered_set<int> vis;
        dfs(0,-1,disc,low,bridges,vis);
        return bridges;
    }
};
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