<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 {
    private:
    int timer=1;
    void dfs( vector<vector<int> >&adj,int cn,int pa,vector<int>&vis,int tin[],int li[], vector<vector<int>>&ans)
    {
        vis[cn]=1;
        tin[cn]=li[cn]=timer;
        timer++;
        for (auto x:adj[cn])
        {
            if(x==pa)
            continue;
            if(vis[x]==0)
            {
                dfs(adj,x,cn,vis,tin,li,ans);
                li[cn]=min(li[cn],li[x]);
                if(li[x]>tin[cn]){
                vector<int>a;
                a.push_back(cn);
                a.push_back(x);
                ans.push_back(a);
                }
            }
            else
            {
                 li[cn]=min(li[cn],li[x]);
            }
        }
    }
public:
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<vector<int> >adj(n+1);
        for(auto x:connections)
        {
            int a=x[0],b=x[1];
            cout<<x[0]<<" "<<x[1];
            adj[a].push_back(b);
            adj[b].push_back(a);
        }
        vector<vector<int>>ans;
        vector<int>vis(n+1,0);
        int tin[n+1],li[n+1];
        dfs(adj,0,-1,vis,tin,li,ans);
        return ans;
    }
};
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