<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:

    vector<vector<int>> ans;

    void dfs(int node, int &timeToReach, int parent, vector<int> &inTime, vector<int> &minNodeTime, vector<vector<int>> &adj) {
        inTime[node] = timeToReach;
        minNodeTime[node] = timeToReach;
        timeToReach++;

        for(int ng: adj[node]) {
            if(ng != parent) {
                if(inTime[ng] == -1) {
                    dfs(ng, timeToReach, node, inTime, minNodeTime, adj);
                    if(minNodeTime[ng] > inTime[node]) {
                        ans.emplace_back(vector<int> {node, ng});
                    }
                }
                minNodeTime[node] = min(minNodeTime[node], minNodeTime[ng]);
            }
        }
    }

    vector<vector<int>> criticalConnections(int n, vector<vector<int>>& connections) {
        vector<int> inTime(n, -1);
        vector<int> minNodeTime(n, INT_MAX);
        vector<vector<int>> adj(n);

        for(vector<int> e: connections) {
            adj[e[0]].emplace_back(e[1]);
            adj[e[1]].emplace_back(e[0]);
        }

        int time = 1;

        dfs(0, time, -1, inTime, minNodeTime, adj);
        return ans;
    }
};
195748