Breadth_First_Search.java

// Java program to print BFS traversal from a given source vertex.
// BFS(int s) traverses vertices reachable from s.
import java.io.*;
import java.util.*;

// This class represents a directed graph using adjacency list
// representation
class Graph
{
    private int V;   // No. of vertices
    private LinkedList<Integer> adj[]; //Adjacency Lists

    // Constructor
    Graph(int v)
    {
        V = v;
        adj = new LinkedList[v];
        for (int i = 0; i < v; ++i)
            adj[i] = new LinkedList();
    }

    // Function to add an edge into the graph
    void addEdge(int v,int w)
    {
        adj[v].add(w);
    }

    // prints BFS traversal from a given source s
    void BFS(int s)
    {
        // Mark all the vertices as not visited(By default
        // set as false)
        boolean visited[] = new boolean[V];

        // Create a queue for BFS
        LinkedList<Integer> queue = new LinkedList<Integer>();

        // Mark the current node as visited and enqueue it
        visited[s] = true;
        queue.add(s);

        while (queue.size() != 0)
        {
            // Dequeue a vertex from queue and print it
            s = queue.poll();
            System.out.print(s + " ");

            // Get all adjacent vertices of the dequeued vertex s
            // If a adjacent has not been visited, then mark it
            // visited and enqueue it
            Iterator<Integer> i = adj[s].listIterator();
            while (i.hasNext())
            {
                int n = i.next();
                if (!visited[n])
                {
                    visited[n] = true;
                    queue.add(n);
                }
            }
        }
    }
}

public class Breadth_First_Search {

    public static void main(String args[])
    {
        int nodes;
        Scanner sc = new Scanner(System.in);
        nodes = sc.nextInt();
        Graph g = new Graph(nodes);

        int edges = sc.nextInt();
        int u, v;
        for (int i = 0; i < edges; i++)
        {
            u = sc.nextInt();
            v = sc.nextInt();
            g.addEdge(u, v);
        }

        System.out.println("Following is Breadth First Traversal (starting from vertex 2)");
        g.BFS(2);
    }
}

/*
INPUT
4
6
0 1
0 2
1 2
2 0
2 3
3 3

OUTPUT
  Following is Breadth First Traversal (starting from vertex 2)
  2 0 3 1
*/