<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:

    vector<vector<int>> ans,G;

    vector<int> tin,low,vis;

    int timer=0;

    void dfs(int node,int par){
        vis[node]=1;
        tin[node]=timer++;
        low[node]=tin[node];
        for(auto ch:G[node]){
            if(ch==par)continue;
            if(vis[ch]){
                low[node]=min(low[node],tin[ch]);
            }
            else{
                dfs(ch,node);
                if(low[ch]>tin[node]){
                    vector<int> temp;
                    temp.push_back(ch);
                    temp.push_back(node);
                    ans.push_back(temp);
                }
                low[node]=min(low[node],low[ch]);
            }
        }
    }


    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        G.resize(n);
        low.resize(n);
        tin.resize(n);
        vis.resize(n,0);
        for(auto ele:connections){
            int a=ele[0];
            int b=ele[1];
            G[a].push_back(b);
            G[b].push_back(a);
        }
        for(int i=0;i<n;++i){
            if(!vis[i])dfs(i,-1);
        }
        return ans;
    }
};
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