<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 dfg(int po,int pr,int de,vector<int>&vis,vector<int> ed[],vector<vector<int>>&ans){
        if(vis[po]!=-1) return po;
        int val=-1,x;vis[po]=de;
        de++;
        for(int i=0;i<ed[po].size();i++){
            if(ed[po][i]==pr) continue;
            x=dfg(ed[po][i],po,de,vis,ed,ans);
            if(x!=-1&&x!=po){
                if(val==-1) {
                    if(vis[x]<vis[po]) val=x;
                } else {
                    if(vis[x]<vis[val]) val=x;
                }
            }
        }
        if(val==-1) ans.push_back ({po,pr});
        return val;
    }
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& con ) {
        vector<int> ed[n];
        for(int i=0;i<con.size();i++){
            ed[con[i][0]].push_back (con[i][1]);
            ed[con[i][1]].push_back (con[i][0]);
        }
        vector<vector<int>> ans;
        vector<int> vis(n,-1);
        int yy=dfg(0,-1,0,vis,ed,ans);
        if(yy==-1)ans.pop_back();
        return ans;
    }
};
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