programming-examples/java/Searching_Sorting_Algorithms/Java Program to Represent Graph Using Incidence List.java

/*This is a java program to represent graph as a incidence list. The incidence matrix of G is a n × m matrix (b_{ij}), where n and m are the numbers of vertices and edges respectively, such that b_{ij} = 1 if the vertex v_i and edge x_j are incident and 0 otherwise. If this relationship is represented by a list, list is known as incidence list.*/

//This is a java program to represent graph as a incidence list
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.LinkedList;
import java.util.Scanner;

public class Represent_Graph_Incidence_List
{
    private Map<Integer, List<Integer>> incidenceList;
    private int v;

    public Represent_Graph_Incidence_List(int vertices)
    {
        v = vertices;
        incidenceList = new HashMap<Integer, List<Integer>>();
        for (int i = 1; i <= v; i++)
            incidenceList.put(i, new LinkedList<Integer>());
    }

    public void setEdge(int to, int from, int edge_number)
    {
        List<Integer> slist = incidenceList.get(to);
        slist.add(edge_number);
        List<Integer> dlist = incidenceList.get(from);
        dlist.add(edge_number);
    }

    public List<Integer> getEdge(int vertex)
    {
        return incidenceList.get(vertex);
    }

    public static void main(String args[])
    {
        int v, e, count = 1, to, from, edgeNumber;
        Scanner sc = new Scanner(System.in);
        Represent_Graph_Incidence_List glist;
        try
            {
                System.out.println("Enter the number of vertices: ");
                v = sc.nextInt();
                System.out.println("Enter the number of edges: ");
                e = sc.nextInt();
                glist = new Represent_Graph_Incidence_List(v);
                System.out
                .println("Enter the edges in the graph : <edgenumber> <to> <from>");
                while (count <= e)
                    {
                        edgeNumber = sc.nextInt();
                        to = sc.nextInt();
                        from = sc.nextInt();
                        glist.setEdge(to, from, edgeNumber);
                        count++;
                    }
                System.out
                .println("The Incidence List Representation of the graph is: ");
                System.out.println("Vertex   EdgeNumber");
                for (int vertex = 1; vertex <= v; vertex++)
                    {
                        System.out.print(vertex + "\t:");
                        List<Integer> edgeList = glist.getEdge(vertex);
                        for (int j = 1;; j++)
                            {
                                if (j != edgeList.size())
                                    System.out.print(edgeList.get(j - 1) + "\t");
                                else
                                    {
                                        System.out.print(edgeList.get(j - 1));
                                        break;
                                    }
                            }
                        System.out.println();
                    }
            }
        catch (Exception E)
            {
                System.out.println("Something went wrong");
            }
        sc.close();
    }
}

/*
Enter the number of vertices:
5
Enter the number of edges:
5
Enter the edges in the graph : <edgenumber> <to> <from>
1 1 2
2 2 4
3 5 4
4 4 3
5 5 1
The Incidence List Representation of the graph is:
Vertex   EdgeNumber
1	:1	5
2	:1	2
3	:4
4	:2	3	4
5	:3	5