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Check if a linked list is Circular Linked List

Given a singly linked list, find if the linked list is circular or not. A linked list is called circular if it not NULL terminated and all nodes are connected in the form of a cycle. Below is an example of circular linked list.

An empty linked list is considered as circular.

Note that this problem is different from , here all nodes have to be part of cycle.



The idea is to store head of the linked list and traverse it. If we reach NULL, linked list is not circular. If reach head again, linked list is circular.

C++

// C++ program to check if linked list is circular
#include<bits/stdc++.h>
using namespace std;
  
/* Link list Node */
struct Node
{
    int data;
    struct Node* next;
};
  
/* This function returns true if given linked
   list is circular, else false. */
bool isCircular(struct Node *head)
{
    // An empty linked list is circular
    if (head == NULL)
       return true;
  
    // Next of head
    struct Node *node = head->next;
  
    // This loop would stope in both cases (1) If
    // Circular (2) Not circular
    while (node != NULL && node != head)
       node = node->next;
  
    // If loop stopped because of circular
    // condition
    return (node == head);
}
  
// Utility function to create a new node.
Node *newNode(int data)
{
    struct Node *temp = new Node;
    temp->data = data;
    temp->next = NULL;
    return temp;
}
  
/* Drier program to test above function*/
int main()
{
    /* Start with the empty list */
    struct Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
  
    isCircular(head)? cout << "Yes " :
                      cout << "No " ;
  
    // Making linked list circular
    head->next->next->next->next = head;
  
    isCircular(head)? cout << "Yes " :
                      cout << "No " ;
  
    return 0;
}

Java

// Java program to check if 
// linked list is circular
import java.util.*;
  
class GFG
{
      
/* Link list Node */
static class Node
{
    int data;
    Node next;
}
  
/*This function returns true if given linked
list is circular, else false. */
static boolean isCircular( Node head)
{
    // An empty linked list is circular
    if (head == null)
    return true;
  
    // Next of head
    Node node = head.next;
  
    // This loop would stope in both cases (1) If
    // Circular (2) Not circular
    while (node != null && node != head)
    node = node.next;
  
    // If loop stopped because of circular
    // condition
    return (node == head);
}
  
// Utility function to create a new node.
static Node newNode(int data)
{
    Node temp = new Node();
    temp.data = data;
    temp.next = null;
    return temp;
}
  
/* Driver code*/
public static void main(String args[])
{
    /* Start with the empty list */
    Node head = newNode(1);
    head.next = newNode(2);
    head.next.next = newNode(3);
    head.next.next.next = newNode(4);
  
    System.out.print(isCircular(head)? "Yes " :
                    "No " );
  
    // Making linked list circular
    head.next.next.next.next = head;
  
    System.out.print(isCircular(head)? "Yes " :
                    "No " );
  
}
}
  
// This code contributed by Arnab Kundu

C#

// C# program to check if
// linked list is circular
using System;
public class GFG
{

/* Link list Node */
public class Node
{
public int data;
public Node next;
}

/*This function returns true if given linked
list is circular, else false. */
static bool isCircular( Node head)
{
// An empty linked list is circular
if (head == null)
return true;

// Next of head
Node node = head.next;

// This loop would stope in both cases (1) If
// Circular (2) Not circular
while (node != null && node != head)
node = node.next;

// If loop stopped because of circular
// condition
return (node == head);
}

// Utility function to create a new node.
static Node newNode(int data)
{
Node temp = new Node();
temp.data = data;
temp.next = null;
return temp;
}

/* Driver code*/
public static void Main(String []args)
{
/* Start with the empty list */
Node head = newNode(1);
head.next = newNode(2);
head.next.next = newNode(3);
head.next.next.next = newNode(4);

Console.Write(isCircular(head)? “Yes ” :
“No ” );

// Making linked list circular
head.next.next.next.next = head;

Console.Write(isCircular(head)? “Yes ” :
“No ” );

}
}
// This code has been contributed by 29AjayKumar


Output :

No
Yes

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This article is attributed to GeeksforGeeks.org

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