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JAVA EXAMPLE PROGRAMS

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Program: Identify loop/cycle in a LinkedList.


Problem Description:

Write a simple code to identify loop or cycle in a linked list.

In general, the last node is null in a Linked list. A loop/cycle exists in a LinkedList when no null is reached as we traverse throughout the LinkedList.

There are various ways to identify loop/cycle in a linked list.

The below diagram illustrates linked list loop/cycle:

Approach 1: Using Hashing

  • Traverse the linked list nodes and keep adding the node addresses in a Hash Table.
  • At any point, if NULL is reached then the loop/cycle doesnot exist, return false.
  • If next of current node points to any of the previously stored nodes in Hash Table, then return true.

The disadvantage with above approach is it requires extra storage to keep track of hashes.

Approach 2: Floyd’s Cycle-Finding Algorithm

  • Traverse linked list nodes using two pointers.
  • Move first pointer by one step and second pointer by two step.
  • If the pointers meet at the same node then there is a loop.
  • At any point, if NULL is reached then the loop/cycle doesnot exist, return false.



IdentifyLinkedListLoop
package com.java2novice.algos;

import java.util.HashSet;
import java.util.Set;

public class IdentifyLinkedListLoop {

    static class Node {
        int data;
        Node next;
        Node(int tmp) {data = tmp;}
    }

    public static void main(String[] a){

        Node n1 = new Node(34);
        Node n2 = new Node(25);
        Node n3 = new Node(31);
        Node n4 = new Node(56);
        Node n5 = new Node(45);
        n1.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n3;

        System.out.println("Is loop detected: "+identifyLoopV2(n1));
    }

    static boolean identifyLoopV1(Node head) {
        /**
         * In this method, we will keep track of traversed node hash and check with each traversing node.
         * If any match found, then we noticed Loop.
         * If we get hash as null, then we didnt find loop.
         */
        Set<Node> hash = new HashSet<>();
        while (head != null){
            if(hash.contains(head)) {
                return Boolean.TRUE;
            }
            hash.add(head);
            head = head.next;
        }
        return Boolean.FALSE;
    }

    static boolean identifyLoopV2(Node head) {

        /**
         * In this method, we will keep track of two pointers.
         * One pointer moves one step at a time and second one two steps.
         * If both pointers are same at any iteration, then the loop exists.
         */
        Node slowMv = head;
        Node fastMv = head;
        while (slowMv != null && fastMv != null && fastMv.next != null) {
            slowMv = slowMv.next;
            fastMv = fastMv.next.next;
            if(slowMv == fastMv) {
                return Boolean.TRUE;
            }
        }
        return Boolean.FALSE;
    }
}

Output:
Is loop detected: true
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List Of All Interview Programs:

  1. Remove last node of the Linked List
  2. Identify middle element of a Linked List
  3. Identify given LinkedList is a palindrom or not using Stack.
  4. Remove duplicates from sorted linked list
  5. Find Nth node from the end of Linked List
  6. Identify loop/cycle in a LinkedList.
  7. Find length of a loop in a LinkedList.
  8. Detect and remove loop in a LinkedList.
  9. How to reverse Singly Linked List?
  10. Check if given Linked List is a Circular Linked List or not.
  11. Find out duplicate number between 1 to N numbers.
  12. Find out middle index where sum of both ends are equal.
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  14. Write a program to create deadlock between two threads.
  15. Write a program to reverse a string using recursive algorithm.
  16. Write a program to reverse a number.
  17. Write a program to convert decimal number to binary format.
  18. Write a program to find perfect number or not.
  19. Write a program to implement ArrayList.
  20. Write a program to find maximum repeated words from a file.
  21. Wrie a program to find out duplicate characters in a string.
  22. Write a program to find top two maximum numbers in a array.
  23. Write a program to sort a map by value.
  24. Write a program to find common elements between two arrays.
  25. How to swap two numbers without using temporary variable?
  26. Write a program to print fibonacci series.
  27. Write a program to find sum of each digit in the given number using recursion.
  28. Write a program to check the given number is a prime number or not?
  29. Write a program to find the given number is Armstrong number or not?
  30. Write a program to convert binary to decimal number.
  31. Write a program to check the given number is binary number or not?
  32. Write a program for Bubble Sort in java.
  33. Write a program for Insertion Sort in java.
  34. Write a program to implement hashcode and equals.
  35. How to get distinct elements from an array by avoiding duplicate elements?
  36. Write a program to get distinct word list from the given file.
  37. Write a program to get a line with max word count from the given file.
  38. Write a program to convert string to number without using Integer.parseInt() method.
  39. Write a program to find two lines with max characters in descending order.
  40. Write a program to find the sum of the first 1000 prime numbers.
  41. Find longest substring without repeating characters.
  42. Write a program to remove duplicates from sorted array.
  43. How to sort a Stack using a temporary Stack?
  44. Write a program to print all permutations of a given string.
  45. Implement Binary Search Tree (BST)
  46. Find min and max value from Binary Search Tree (BST)
  47. Find height of a Binary Search Tree (BST)
  48. Implement Binary Search Tree (BST) Level order traversal (breadth first).
  49. Implement Binary Search Tree (BST) pre-order traversal (depth first).
  50. Implement Binary Search Tree (BST) in-order traversal (depth first).
  51. Implement Binary Search Tree (BST) post-order traversal (depth first).
  52. How to check the given Binary Tree is Binary Search Tree (BST) or not?
  53. How to delete a node from Binary Search Tree (BST)?
  54. Write a program to find common integers between two sorted arrays.
  55. Write a program to find given two trees are mirror or not.
  56. HackerRank stack problem - Find maximum element.
  57. HackerRank stack problem - Balanced Brackets.
  58. HackerRank stack problem - Equal Stacks.
  59. HackerRank stack problem - Game Of Two Stacks.
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I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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