<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:
    static constexpr int kInvalidRank = -1;
    struct node
    {
        int rank = kInvalidRank;
        vector<int> neighbours;
    };

    static constexpr auto hash = [](const auto& v) {
        std::hash<int> hasher;
        size_t seed = 0;
        for (int i : v) {
            seed ^= hasher(i) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
        }
        return seed;
    };
    using EdgesSet = unordered_set<vector<int>, decltype(hash)>;

    int dfs(vector<node>& nodes, EdgesSet& edges, int current, int rank) {
        if (nodes[current].rank != kInvalidRank) {
            return nodes[current].rank;
        }
        nodes[current].rank = rank;

        int minRank = rank + 1;
        for (int neighbour : nodes[current].neighbours) {
            // Skip the parent node, we can't have a cycle of only two nodes
            if (nodes[neighbour].rank != kInvalidRank && nodes[neighbour].rank == rank - 1) {
                continue;
            }

            int neighbourRank = dfs(nodes, edges, neighbour, rank + 1);
            // Remove the neighbour edge in case we detect a cycle
            if (neighbourRank <= rank) {
                edges.erase({min(current, neighbour), max(current, neighbour)});
            }

            minRank = min(minRank, neighbourRank);
        }

        return minRank;
    }

    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<node> nodes(n);
        EdgesSet edges;

        // Build an adjacency list
        for (const auto& edge : connections) {
            nodes[edge[0]].neighbours.push_back(edge[1]);
            nodes[edge[1]].neighbours.push_back(edge[0]);
            edges.insert({min(edge[0], edge[1]), max(edge[0], edge[1])});
        }

        dfs(nodes, edges, 0, 0);

        return {edges.begin(), edges.end()};
    }
};
212916