Question

Write a C# function bool HasCycle(Node head) that detects a cycle in a singly linked list.
It must return a boolean true if the list contains a cycle, or false otherwise. HasCycle
has a reference to a Node object, that points to the head of a linked list, as parameter.

Answer 1

TO detect Loop in linked list we can this algorithm

Floyd’s Cycle-Finding Algorithm:

• Traverse linked list using two pointers.
• Move one pointer( p_slow )by one and another pointer(p_fast) by two.
• If these pointers meet at the same node then there is a loop.
• Else If pointers do not meet then linked list doesn’t have a loop

So Now try to code is in C#

Function is boolHasCycle( Node Head) takes head of the linked list as argument  

This function will return true if cycle is detected else false

Code is

bool HasCycle(Node head)
   {
       Node p_slow = head, p_fast = head;
       while (p_slow != null && p_fast != null &&
                               p_fast.next != null)
       {

// increment p_slow by one
           p_slow = p_slow.next;

// increment p_fast by two
           p_fast = p_fast.next.next;

// when they are equal or pointing to same node that means cycle is detected
           if (p_slow == p_fast)
           {
               return true;
           }
       }
       return false;
   }

Full Code for Complete Implementation of above function

using System;

public class LinkedList
{
   Node head; // head of list

   /* Linked list Node*/
   public class Node
   {
       public int data;
       public Node next;
       public Node(int d)
       {
           data = d;
           next = null;
       }
   }

   /* This function will Inserts a new Node at beginning of the list. */
   public void push(int new_data)
   {
       /* 1 & 2: Allocate the memory to Node &
               Put in the data*/
       Node new_node = new Node(new_data);

       // connect new node with head
       new_node.next = head;

       //Make Head to point to new node
  
       head = new_node;
   }
// This function is used to detect cycle in linked list
bool HasCycle(Node head)
   {
       Node p_slow = head, p_fast = head;
       while (p_slow != null && p_fast != null &&
                               p_fast.next != null)
       {
           p_slow = p_slow.next;
           p_fast = p_fast.next.next;
           if (p_slow == p_fast)
           {
          
               return true;
           }
       }
       return false;
   }

   public static void Main(String []args)
   {
       LinkedList llist = new LinkedList();
       bool found;

       llist.push(20);
       llist.push(4);
       llist.push(15);
       llist.push(10);

       /*Delibrately creating a loop in the list for testing */
      llist.head.next.next.next.next = llist.head;

       found=llist.HasCycle(llist.head);
       if(found)
       Console.WriteLine("Found loop in Linked List");
       else
       Console.WriteLine("No loop is found in Linked List");
      
   }
}

Output is

Found loop in Linked List

Input Code Screenshot

{ { { } } 1 using System; 2 3 public class LinkedList 4 5 Node head; // head of list 6 7 /* Linked list Node*/ 8 public class Node 9. 10 public int data; 11 public Node next; 12 public Node(int d) 13 14 data = d; 15 next = null; 16 17 18 19 /* This function will Inserts a new Node at beginning of the list. */ 20 public void push(int new_data) 21 - 22 - /* 1 & 2: Allocate the memory to Node & 23 Put in the data*/ 24 Node new_node = new Node(new_data); 25 26 // connect new node with head 27 new_node.next = head; 28 29 //Make Head to point to new node 30 31 head = new_node; 32 33 34 hool Hasſvcle/Node head) { }

new_node.next = head; //Make Head to point to new node head = new_node; } bool HasCycle(Node head) { Node p_slow = head, p_fast = head; while (p_slow != null && p_fast != null && p_fast.next != null) { p_slow p_slow.next; p_fast p_fast.next.next; if (p_slow == p_fast) { 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43- 44 45 46 47 48 49 50 51 52. 53 54 55 56 57 58 59 60 return true; } } return false; perlen public static void Main(String []args) { Linkedlist llist = new Linkedlist(); bool found; llist.push(20); llist.push(4); llist.push(15); llist.push(10);

40 41 p_slow = p_slow.next; p_fast.next.next; if (p_slow == p_fast) p_fast 42 { 43 44 45 return true; 46 } } return false; } public static void Main(String []args) { Linkedlist llist = new linkedList(); bool found; llist.push(20); llist.push(4); llist.push(15); llist.push(10); 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 } 72 73 /*Delibrately creating a loop for testing */ llist.head.next.next next next = llist.head; found=llist.HasCycle(llist.head); if(found) Console.WriteLine("Found loop in Linked List"); else Console.WriteLine("No loop is found in Linked list"); }

So this is how we can check cycle in linked list using C#

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