Java Program to Detect loop in a LinkedList

In this article, we will understand how to detect loop in a LinkedList. A linked list is a sequence of data structures, which are connected together via links. Linked List is a sequence of links which contains items. Each link contains a connection to another link.

Below is a demonstration of the same −

Suppose our input is −

Run the program

The desired output would be −

The loop exists in the linked list

Algorithm

Step 1 - START
Step 2 - Declare namely
Step 3 - Define the values.
Step 4 - Define the class with the relevant members.
Step 5 - Create an instance of the class, and initialize the nodes.
Step 6 - Define functions to check if it is a loop.
Step 7 - For this, create a HashSet, and add elements to the topmost node.
Step 8 - Point the node to the next element after every iteration.
Step 9 - In the main method, create an instance and add elements to the list using the ‘push’ method.
Step 10 - Call the ‘check_loop’ method, and display the relevant message on the console.
Step 11 - Stop

Example 1

Here, we use the traversal method to find the loop.

import java.util.*;
public class Demo {
   static Node head;
   static class Node {
      int data;
      Node next;
      Node(int d){
         data = d;
         next = null;
      }
   }
   static public void push(int new_data){
      Node new_node = new Node(new_data);
      new_node.next = head;
      head = new_node;
   }
   static boolean check_loop(Node head){
      HashSet<Node> s = new HashSet<Node>();
      while (head != null) {
         if (s.contains(head))
            return true;
         s.add(head);
         head = head.next;
      }
      return false;
   }
   public static void main(String[] args){
      System.out.println("The required packages have been imported");
      Demo input_list = new Demo();
      input_list.push(45);
      input_list.push(60);
      input_list.push(75);
      input_list.push(90);
      input_list.head.next.next.next.next = input_list.head;
      if (check_loop(head))
         System.out.println("The loop exists in the linked list");
      else
         System.out.println("The loop doesnot exists in the linked list");
      }
   }

Output

The required packages have been imported
The loop exists in the linked list

Example 2

Here, we encapsulate the operations into functions exhibiting object oriented programming.

public class Demo {
Node head;
static class Node {
int value;
Node next;
Node(int d) {
value = d;
next = null;
}
}
public boolean check_loop() {
Node first_node = head;
Node second_node = head;
while(first_node != null && first_node.next !=null) {
first_node = first_node.next.next;
second_node = second_node.next;
if(first_node == second_node) {
return true;
}
}
return false;
}
public static void main(String[] args) {
Demo input_list = new Demo();
input_list.head = new Node(45);
Node second_node = new Node(60);
Node third_node = new Node(75);
Node fourth_node = new Node(90);
input_list.head.next = second_node;
second_node.next = third_node;
third_node.next = fourth_node;
fourth_node.next = second_node;
System.out.print("The elements of the linked list are: ");
int i = 1;
while (i <= 4) {
System.out.print(input_list.head.value + " ");
input_list.head = input_list.head.next;
i++;
}
boolean loop = input_list.check_loop();
if(loop) {
System.out.println("\nThere is a loop in the linked list.");
}
else {
System.out.println("\nThere is no loop in the linked list.");
}
}
}

Output

The required packages have been imported
The loop exists in the linked list