<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 dfs(vector<vector<int>> &graph, int i, int parent, int &currank, vector<int> &rank, vector<vector<int>> &ans) {
        rank[i] = currank;
        int curmin = currank++;
        for (auto j : graph[i]) {
            if (j == parent) {
                continue;
            }
            if (rank[j] == -1) {
                int tmp = dfs(graph, j, i, currank, rank, ans);
                if (tmp > rank[i]) {
                    ans.push_back({i, j});
                }
                curmin = min(curmin, tmp);
            }
            else {
                curmin = min(curmin, rank[j]);
            }             
        }
        return curmin;
    }

    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<vector<int>> graph(n);
        for (auto &cnt : connections) {
            graph[cnt[0]].push_back(cnt[1]);
            graph[cnt[1]].push_back(cnt[0]);
        }
        vector<int> rank(n, -1);
        vector<vector<int>> ans;
        int currank = 0;
        dfs(graph, 0, -1, currank, rank, ans);
        return ans;
    }
};
168771