Question

Implement Singly Linked List detectLoop in Java.

It would check whether the linked list contains a loop, return true if there exist a loop, false if not.

Answer 1

// Java program to detect a loop in a linked list

import java.util.*;

  

public class LinkedList {

  

    static Node head; // head of list

  

    /* Linked list Node*/

    static class Node {

        int data;

        Node next;

        Node(int d)

        {

            data = d;

            next = null;

        }

    }

  

    /* Inserts a new Node at front of the list. */

    static public void push(int new_data)

    {

        /* 1 & 2: Allocate the Node &

                  Put in the data*/

        Node new_node = new Node(new_data);

  

        /* 3. Make next of new Node as head */

        new_node.next = head;

  

        /* 4. Move the head to point to new Node */

        head = new_node;

    }

  

    // Returns true if there is a loop in linked

    // list else returns false.

    static boolean detectLoop(Node h)

    {

        HashSet<Node> s = new HashSet<Node>();

        while (h != null) {

            // If we have already has this node

            // in hashmap it means their is a cycle

            // (Because you we encountering the

            // node second time).

            if (s.contains(h))

                return true;

  

            // If we are seeing the node for

            // the first time, insert it in hash

            s.add(h);

  

            h = h.next;

        }

  

        return false;

    }

  

    /* Driver program to test above function */

    public static void main(String[] args)

    {

        LinkedList llist = new LinkedList();

  

        llist.push(20);

        llist.push(4);

        llist.push(15);

        llist.push(10);

  

        /*Create loop for testing */

        llist.head.next.next.next.next = llist.head;

  

        if (detectLoop(head))

            System.out.println("Loop found");

        else

            System.out.println("No Loop");

    }

}