<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:
    void dfs(int v, int p, int& t, int minT[], int discT[], std::vector<std::vector<int>>& adj, std::vector<std::vector<int>>& ans) {
        minT[v] = discT[v] = ++t;
        for (auto& u : adj[v]) {
            if (u == p) continue;
            if (!discT[u]) {
                dfs(u, v, t, minT, discT, adj, ans);
                minT[v] = std::min(minT[v], minT[u]);
                if (discT[v] < minT[u]) ans.push_back({v, u});
            }
            else minT[v] = std::min(minT[v], discT[u]);
        }
    }
public:
    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        std::vector<std::vector<int>> ans;
        std::vector<std::vector<int>> adj(n);
        int minT[n], discT[n];
        std::memset(minT, 0, sizeof minT);
        std::memset(discT, 0, sizeof discT);
        for (auto& conn : connections) {
            adj[conn[0]].push_back(conn[1]);
            adj[conn[1]].push_back(conn[0]);
        }
        int t = 0;
        dfs(0, 0, t, minT, discT, adj, ans);
        return ans;
    }
};
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