Check if a linked list is Circular Linked List

Difficulty Level : Easy
Last Updated : 07 Jul, 2021

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

An empty linked list is considered as circular.
Note that this problem is different from cycle detection problem, 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 stop 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;
}
 
/* Driver 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\n" :
                      cout << "No\n" ;
 
    // Making linked list circular
    head->next->next->next->next = head;
 
    isCircular(head)? cout << "Yes\n" :
                      cout << "No\n" ;
 
    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 stop 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\n" :
                    "No\n" );
 
    // Making linked list circular
    head.next.next.next.next = head;
 
    System.out.print(isCircular(head)? "Yes\n" :
                    "No\n" );
 
}
}
 
// This code contributed by Arnab Kundu

Python3

# A simple Python program to check if a linked list is circular
 
# Node class
class Node:
 
    # Function to initialise the node object
    def __init__(self, data):
        self.data = data # Assign data
        self.next = None # Initialize next as null
 
 
# Linked List class contains a Node object
class LinkedList:
 
    # Function to initialize head
    def __init__(self):
        self.head = None
 
def Circular(head):
    if head==None:
        return True
         
    # Next of head
    node = head.next
    i = 0
     
    # This loop would stop in both cases (1) If
    # Circular (2) Not circular
    while((node is not None) and (node is not head)):
        i = i + 1
        node = node.next
     
    return(node==head)
 
 
# Code execution starts here
if __name__=='__main__':
    llist = LinkedList()
    llist.head = Node(1)
    second = Node(2)
    third = Node(3)
    fourth = Node(4)
     
    llist.head.next = second;
    second.next = third;
    third.next = fourth
     
    if (Circular(llist.head)):
        print('Yes')
    else:
        print('No')
     
    fourth.next = llist.head
     
    if (Circular(llist.head)):
        print('Yes')
    else:
        print('No')
         
# This code is contributed by Sanket Badhe

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 stop 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\n" :
                    "No\n" );
 
    // Making linked list circular
    head.next.next.next.next = head;
 
    Console.Write(isCircular(head)? "Yes\n" :
                    "No\n" );
 
}
}
// This code has been contributed by 29AjayKumar

Javascript

<script>
// javascript program to check if
// linked list is circular
 
    /* Link list Node */
class Node {
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}
 
 
    /*
     * This function returns true if given linked list is circular, else false.
     */
    function isCircular( head) {
        // An empty linked list is circular
        if (head == null)
            return true;
 
        // Next of head
         node = head.next;
 
        // This loop would stop 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.
    function newNode(data) {
         temp = new Node();
        temp.data = data;
        temp.next = null;
        return temp;
    }
 
    /* Driver code */
     
        /* Start with the empty list */
         head = newNode(1);
        head.next = newNode(2);
        head.next.next = newNode(3);
        head.next.next.next = newNode(4);
 
        document.write(isCircular(head) ? "Yes<br/>" : "No<br/>");
 
        // Making linked list circular
        head.next.next.next.next = head;
 
        document.write(isCircular(head) ? "Yes<br/>" : "No<br/>");
 
// This code contributed by gauravrajput1
</script>

Output :

No
Yes

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This article is contributed by Shivam Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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