eMahtab / tarjans-algorithm

Tarjan's Algorithm : O(V + E)

Tarjan's algorithm is used to find the strongly connected components in a graph using DFS.

Strongly Connected Component :

If we can reach every vertex of a component from every other vertex in that component then it is called a Strongly Connected Component (SCC).

Algorithm :

1. Initialize both discovery and low array with -1, note that discovery array will serve two purposes : if discovery[vertex] == -1 it means this vertex was not discovered before and second purpose is we will store the discovery id of vertex at discovery[vertex].
2. Take a parents array and initialize all the entries with -1. parents array will be used so that while doing dfs for neighbor v from parent u, we don't consider the parent node u. Before calling the DFS on neighbor v we will set parents[v] = u
3. Take a variable which denotes the id at which a vertex was first discovered and initialize it to 0
4. Start the DFS from vertex 0
5. If the vertex is not already discovered then set discovery[vertex] and low[vertex] to be id and increment id by 1 for next node, if a vertex is already discovered return from dfs().
6. Next check all the neighbors of this vertex (those which are not yet discovered and also those which are already discovered).

(i). For a neighbor which was not already discovered, set the parents[v]=u and call dfs(), when dfs is completed for neighbor v, we update low[u] as minimum of low[u] and low[v] ; low[u] = Math.min(low[u], low[v]); also we check if discovery[u] < low[v] then it means (u,v) is a critical edge

(ii). For a neighbor which was already discovered and which is not parents of this vertex, update low[u] as minimum of discovery[v] and low[u] ; low[u] = Math.min(low[u], discovery[v]);

References :

1. Tarjan's Algorithm : https://www.youtube.com/watch?v=RYaakWv5m6o
2. Critical connections in a network : https://github.com/eMahtab/critical-connections-in-a-network