Java program to detect loop in a LinkedList

What is a LinkedList?

A Linked list is a sequence of data structures where each node contains two parts as follows -

• Data: The value or information stored in the node.
• Next: A reference (or pointer) to the next node in the sequence.

Detecting a Loop in a LinkedList

A loop or cycle means that the last node of a linked list is connected back to one of the nodes earlier in the list, creating a cycle. Or, the next pointer of a node points to itself. The following are the ways to detect a loop or cycle in a linked list:

• Using HashSet
• Using Floyd's Cycle-Finding Algorithm

Using HashSet

The HashSet class in Java implements the Set interface; it internally uses a Hash Table to store data. To detect loops in a LinkedList.

• We need to keep track of the nodes we have already visited by storing them in a HashSet object.
• As we traverse the linked list, we need to check if the current node is already in the HashSet.
• If it exists, then we have found a loop.

Example

In the following example, we will create a linked list and then use a HashSet to detect if there is a loop or cycle in the linked list.

import java.util.HashSet;
import java.util.Set;
public class DetectLoop {
   static class Node {
      int data;
      Node next;

      Node(int data) {
         this.data = data;
         this.next = null;
      }
   }

   static boolean checkLoop(Node head) {
      Set<Node> visitedNode = new HashSet<>();
      Node current = head;
      
      while (current != null) {
         if(visitedNode.contains(current)){
            return true;
         }
         visitedNode.add(current);
         current = current.next;
      }
      return false;
   }
   public static void main(String[] args) {
      Node head = new Node(1);
      head.next = new Node(2);
      head.next.next = new Node(3);
      head.next.next.next = new Node(4);
      head.next.next.next.next = head.next;

      if (checkLoop(head)) {
         System.out.println("Loop detected in the linked list.");
      } else {
         System.out.println("No loop detected in the linked list.");
      }
   }
}

Output

Following is the output of the above code:

Loop detected in the linked list.

Using Floyd's Cycle-Finding Algorithm

Floyd's Cycle-Finding Algorithm, also known as the Tortoise and Hare algorithm. It uses two pointers fast pointer and a slow pointer. The slow pointer moves one step at a time, while the fast pointer moves two steps at a time. If there is a loop, the fast pointer will eventually meet the slow pointer. Following are the steps -

• Initialize two pointers, slow and fast, both pointing to the head of the linked list.
• Move the slow pointer one step forward and the fast pointer two steps forward in each iteration.
• If the slow pointer and fast pointer meet at any point, then there is a loop in the linked list.
• If the fast pointer reaches the end of the linked list (i.e., fast or fast.next is null), then there is no loop.

Example

In the following example, we will create a linked list and then use Floyd's Cycle-Finding Algorithm to detect if there is a loop in the linked list.

public class DetectLoop {
   static class Node {
      int data;
      Node next;
      Node(int data) {
         this.data = data;
         this.next = null;
      }
   }

   static boolean checkLoop(Node head){
      Node slow = head;
      Node fast = head;
      while(fast != null && fast.next != null){
         slow = slow.next;
         fast = fast.next.next;

         if(slow == fast){
            return true;
         }
      }
      return false;
   }
   public static void main(String[] args){
       Node head = new Node(1);
       head.next = new Node(2);
       head.next.next = new Node(3);
       head.next.next.next = new Node(4);
       head.next.next.next.next = head.next;
       
       if (checkLoop(head)) {
           System.out.println("Loop detected in the linked list.");
        } else {
            System.out.println("No loop detected in the linked list.");
        }
    }
}

Output

Following is the output of the above code:

Loop detected in the linked list.