GRAPH REPRESENTATION VIA ADJACENCY LIST IN JAVA & ADD EDGES

Method 1: Add manually

import java.util.*;

public class Wind {

    public static void main(String[] args) {

        HashMap<Integer, ArrayList<Integer>> adjList = new HashMap<>();

        // Adding nodes to the graph

        for (int i = 0; i < 4; i++) {

            adjList.put(i, new ArrayList<>());

        }

        // Adding edges to the graph

        addEdge(adjList, 0, 1);

        addEdge(adjList, 1, 3);

        addEdge(adjList, 1, 2);

        addEdge(adjList, 0, 2);

        // Print the adjacency list

        System.out.println("Adjacency List: " + adjList);

    }

    // Helper method to add an edge to the graph

    public static void addEdge(HashMap<Integer, ArrayList<Integer>> adjList, int source, int destination) {

        adjList.get(source).add(destination);

        adjList.get(destination).add(source); // If the graph is undirected

    }

}

Method 2: Add when edges pre-given in form of 2-d matrix

import java.util.*;

public class Wind {

    public static void main(String[] args) {

        HashMap<Integer, ArrayList<Integer>> adjList = new HashMap<>();

        // Adding nodes to the graph

        for (int i = 0; i < 4; i++) {

            adjList.put(i, new ArrayList<>());

        }

       

        int prerequisite[][] = {{0,1},{1,3},{1,2},{0,2}};

        for (int[] row : prerequisite)

        {

            int v=row[1];

            int u=row[0];

            addEdge(adjList,u,v); // u --> v

        }

        /*for (int[] row : prerequisite) {

            for (int element : row) {

                // Access the current element

                System.out.print(element + " ");

            }

            System.out.println(); // Move to the next line after each row

        }*/

        // Print the adjacency list

        System.out.println("Adjacency List: " + adjList);

    }

    public static void addEdge(HashMap<Integer, ArrayList<Integer>> adjList, int source, int destination) {

        adjList.get(source).add(destination);

        //adjList.get(destination).add(source); // If the graph is undirected

    }

}