Is the implementation for Graph using Adjacency list correct?

Question

I am a beginner with DSA, Since the last couple of days, I was trying to find the correct implementation for the Graph using the adjacency list.

Below I provided the entire code for the way I thought the adjacency list should be implemented.

I have created a SinglyLinkedlist from scratch. And I am using a Hashmap to improve the Time complexity.

The Integer key in the Hashmap acts as the VERTICE & consists of a LinkedList in its VALUE.

In the vertices, I am storing the Integer ID and in the LinkedList, I am storing all the Friend names for that particular ID.

The Graph has 3 methods:

1. InsertVertice(int ID ) - creates an empty LinkedList at given ID in hashmap.

2. insertDataAtID (int ID, String data) - inserts the Data into the LinkedList at the given ID.

3. printAllDataAtID(int ID) - prints all the friend names or Data present in a LinkedList at a given ID/key in the Hashmap.

Can you please go through the Implementation and advice of any mistakes? Or better some suggestions on how an Adjacency list can be implemented more effectively?

Thank you for this effort.

---

import java.util.HashMap;

public class demo {

    static HashMap<Integer,SinglyLinkedlist> graph = new HashMap<>();


    public static void main(String[] args){

      Graph graph = new Graph();


      graph.insertVertice(101);
      graph.insertDataAtID(101,"Deepesh");
      graph.insertDataAtID(101,"Kiran");
      graph.insertDataAtID(101,"Aryan");


      graph.insertVertice(201);
      graph.insertDataAtID(201,"Carl");
      graph.insertDataAtID(201,"Arun");
      graph.insertDataAtID(201,"Kishan");
      graph.insertDataAtID(201,"Betth");


      graph.printAllDataAtID(101);
      graph.printAllDataAtID(201);


    }

}

---

import java.util.HashMap;

public class Graph{

     HashMap<Integer,SinglyLinkedlist> maplist = new HashMap<>();


    void insertVertice(Integer id ){

        maplist.put(id,new SinglyLinkedlist());

    }

     void insertDataAtID(Integer id, String data){

        if (maplist.get(id)==null){
            System.out.println("No such Vertice exist with id : " + id);
            System.out.println("Create the Vertice first by calling insertVertice() method.");
        }

        SinglyLinkedlist linkedlist = maplist.get(id);
        linkedlist.insertNode(data);

    }


     void printAllDataAtID(Integer id) throws NullPointerException {
         if (maplist.get(id) == null) {
             System.out.println("No such Vertice exist with id : " + id);
             System.out.println("Create the Vertice first by calling insertVertice() method.");
         } else {

             SinglyLinkedlist linkedlist = maplist.get(id);
             linkedlist.printAll();

         }
     }

}

---

public class SinglyLinkedlist {

    Node head;
    Node tail;

    public static class Node {
        Node next;
        String data;
    }



    void insertNode(String data) {

        Node newNode = new Node();
        newNode.data = data;

        if (head == null) {
            head = tail = newNode;
            newNode.next = null;
        } else {

            Node temp = head;

            while (temp.next != null) {
                temp = temp.next;
            }

            temp.next = newNode;
            newNode.next = null;
            tail = newNode;

        }
    }


    void removeLastNode() {
        Node temp = head;

        while (temp.next.next != null) {
            temp = temp.next;
        }

        Node removedNode = temp.next;
        tail = temp;
        tail.next = null;

        System.out.println("Removed value : " + removedNode.data);

    }


    void printAll() {
        if (head == null) {
            System.out.println("List is Empty !");
        } else {
            Node temp = head;

            while (temp != null) {
                System.out.print(temp.data + "  ");
                temp = temp.next;
            }
        }
    }


    boolean search(String data) {
        if (head == null) {
            System.out.println("List is Empty !");
        } else {
            Node temp = head;

            while (temp != null) {

                if (temp.data.equals(data)) {
                    System.out.println("Value found !");
                    return true;
                }
                temp = temp.next;

            }

            System.out.println("Value not found !");

        }
        return false;
    }
}

Answer 1

You don't need to traverse the list to insert if you are maintaining tail node.

void insertNode(String data) {
    Node newNode = new Node();
    newNode.data = data;
    if (head == null) {
        head = tail = newNode;
        newNode.next = null;
    } else {
        tail.next = newNode;
        newNode.next = null;
        tail = tail.next;
    }
}