Java Program to Test Using DFS Whether a Directed Graph is Strongly Connected or Not

This is a java program to test whether a directed graph is strongly connected or not. The graph is strongly connected if it has only one connected component.

Here is the source code of the Java Program to Test Using DFS Whether a Directed Graph is Strongly Connected or Not. The Java program is successfully compiled and run on a Windows system. The program output is also shown below.

package com.maixuanviet.graph;
 
import java.util.*;
 
public class StronglyConnectedGraph
{
    private int                 V;
    private int                 preCount;
    private int[]               low;
    private boolean[]           visited;
    private List<Integer>[]     graph;
    private List<List<Integer>> sccComp;
    private Stack<Integer>      stack;
 
    /** function to get all strongly connected components **/
    public List<List<Integer>> getSCComponents(List<Integer>[] graph)
    {
        V = graph.length;
        this.graph = graph;
        low = new int[V];
        visited = new boolean[V];
        stack = new Stack<Integer>();
        sccComp = new ArrayList<>();
        for (int v = 0; v < V; v++)
            if (!visited[v])
                dfs(v);
        return sccComp;
    }
 
    /** function dfs **/
    public void dfs(int v)
    {
        low[v] = preCount++;
        visited[v] = true;
        stack.push(v);
        int min = low[v];
        for (int w : graph[v])
        {
            if (!visited[w])
                dfs(w);
            if (low[w] < min)
                min = low[w];
        }
        if (min < low[v])
        {
            low[v] = min;
            return;
        }
        List<Integer> component = new ArrayList<Integer>();
        int w;
        do
        {
            w = stack.pop();
            component.add(w);
            low[w] = V;
        }
        while (w != v);
        sccComp.add(component);
    }
 
    @SuppressWarnings("unchecked")
    public static void main(String[] args)
    {
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter number of Vertices");
        /** number of vertices **/
        int V = scan.nextInt();
        /** make graph **/
        List<Integer>[] g = new List[V];
        for (int i = 0; i < V; i++)
            g[i] = new ArrayList<Integer>();
        /** accept all edges **/
        System.out.println("Enter number of edges");
        int E = scan.nextInt();
        /** all edges **/
        System.out.println("Enter the edges in the graph : <from> <to>");
        for (int i = 0; i < E; i++)
        {
            int x = scan.nextInt();
            int y = scan.nextInt();
            g[x].add(y);
        }
        StronglyConnectedGraph t = new StronglyConnectedGraph();
        System.out.print("The graph is strongly connected? : ");
        /** print all strongly connected components **/
        List<List<Integer>> scComponents = t.getSCComponents(g);
        Iterator<List<Integer>> iterator = scComponents.iterator();
        boolean stronglyConnected = true;
        while (iterator.hasNext())
        {
            if (iterator.next().size() <= 1)
            {
                stronglyConnected = false;
            }
        }
        System.out.println(stronglyConnected);
        scan.close();
    }
}

Output:

$ javac StronglyConnectedGraph.java
$ java StronglyConnectedGraph
 
Enter number of Vertices
6
Enter number of edges
7
Enter the edges in the graph : <from> <to>
0 1
1 2
1 3
3 4
4 5
5 3
5 2
The graph is strongly connected? : false
 
Enter number of Vertices
8
Enter number of edges
14
Enter the edges in the graph : <from> <to>
0 1
1 2
2 3
3 2
3 7
7 3
2 6
7 6
5 6
6 5
1 5
4 5
4 0
1 4
The graph is strongly connected? : true