<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 low[100005];
    int time[100005];
    int vis[100005];
    void check(unordered_map<int,vector<int>>&m, int idx, vector<vector<int>>&ans, int curr, int par){

        vis[idx] = 1;
        low[idx] = curr;
        time[idx] = curr;
        for(auto j : m[idx]){
            if(j==par) continue;
            if(vis[j]==0){
                check(m, j, ans, curr+1, idx);
            }
            low[idx] = min( low[idx], low[j]);

            if(time[idx] < low[j]){
                vector<int>t = {idx, j};
                ans.push_back(t);
            }
        }

    }

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

        for(auto i : connections){
            m[i[0]].push_back(i[1]);
            m[i[1]].push_back(i[0]);
        }
        memset(low, 0, sizeof(low));
        memset(time, 0, sizeof(time));
        memset(vis, 0, sizeof(vis));
        vector<vector<int>>ans;
        for(int i = 0;i<n;i++){
            if(vis[i]==0){
            check(m, 0, ans, 1, -1);   
            }
        }
        return ans;
    }
};
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