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 is not NULL-terminated and all nodes are connected in the form of a cycle. Below is an example of a circular linked list.

Note:

• An empty linked list is considered circular.
• 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 iterator reaches NULL, linked list is not circular. else If it reaches head again, then linked list is circular. 

Follow the given steps to solve the problem:

• Declare a Node pointer node and initialize it to the head’s next
• Move node pointer to the next node, while the node is not equal to nullptr and node is not equal to the head
• After coming out of the loop, check if the node is equal to head then return true, else return false

Below is the Implementation of the above approach:

C

// C program to check if the linked list is circular
#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node* next;
};

int isCircular(struct Node* head)
{
    // If linked list is empty it is circular
    if (head == NULL)
        return 1;
    struct Node* ptr;
    ptr = head->next;
    // Traversing linked list till last node
    while (ptr != NULL && ptr != head)
        ptr = ptr->next;
    // Condition for circular linked list
    return (ptr == head);
}
// Function to create new Node
struct Node* newnode(int data)
{
    struct Node* temp;
    temp = (struct Node*)malloc(sizeof(struct Node));
    temp->data = data;
    temp->next = NULL;
    return temp;
}

// Driver's code
int main()
{
    // code
    // Starting with empty list
    struct Node* head = newnode(1);
    head->next = newnode(2);
    head->next->next = newnode(3);
    head->next->next->next = newnode(4);

    // Checking for circular list
    if (isCircular(head))
        printf("Yes\n");
    else
        printf("No\n");

    // If not circular making it circular
    head->next->next->next->next = head;
    if (isCircular(head))
        printf("Yes\n");
    else
        printf("No\n");
    return 0;
}

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's code
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

# Python3 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)


# Driver's code
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's 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

// 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

Output

No
Yes

Time Complexity: O(N)
Auxiliary Space: O(1)

Another Approach:

Below is the Implementation of the above approach:

C++

#include <iostream>

using namespace std;

class LinkedList {
private:
    class Node {
    public:
        int data;
        Node* next;

        Node(int data) {
            this->data = data;
            this->next = nullptr;
        }
    };

    Node* head;

public:
    LinkedList() {
        head = nullptr;
    }

    void addToFront(int data) {
        Node* newNode = new Node(data);
        newNode->next = head;
        head = newNode;
    }

    bool isCircular() {
        if (head == nullptr) {
            return false;
        }
        Node* slow = head;
        Node* fast = head->next;
        while (fast != nullptr && fast->next != nullptr) {
            if (slow == fast) {
                return true;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        return false;
    }
};

int main() {
    LinkedList list;
    list.addToFront(1);
    list.addToFront(2);
    list.addToFront(3);
    list.addToFront(4);
    cout << boolalpha << list.isCircular() << endl;
    return 0;
}

Java

import java.util.NoSuchElementException;

public class LinkedList {
    private static class Node {
        public int data;
        public Node next;

        public Node(int data) {
            this.data = data;
            this.next = null;
        }
    }

    private Node head;

    public LinkedList() {
        head = null;
    }

    public void addToFront(int data) {
        Node newNode = new Node(data);
        newNode.next = head;
        head = newNode;
    }

    public boolean isCircular() {
        if (head == null) {
            return false;
        }
        Node slow = head;
        Node fast = head.next;
        while (fast != null && fast.next != null) {
            if (slow == fast) {
                return true;
            }
            slow = slow.next;
            fast = fast.next.next;
        }
        return false;
    }



    public static void main(String[] args) {
        LinkedList list = new LinkedList();
        list.addToFront(1);
        list.addToFront(2);
        list.addToFront(3);
        list.addToFront(4);
        System.out.println(list.isCircular());  // Output: false
    }
}

Python3

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    def add_to_front(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    def is_circular(self):
        if self.head is None:
            return False
        slow = self.head
        fast = self.head.next
        while fast is not None and fast.next is not None:
            if slow == fast:
                return True
            slow = slow.next
            fast = fast.next.next
        return False

list = LinkedList()
list.add_to_front(1)
list.add_to_front(2)
list.add_to_front(3)
list.add_to_front(4)
print(list.is_circular()) # Output: False

C#

// C# code for above approach
using System;

public class Node{
    public int data;
    public Node next;
    public Node(int item){
        data = item;
        next = null;
    }
}

public class GFG{
    public static bool isCircular(Node head){
        if(head == null) return false;

        Node slow = head;
        Node fast = head.next;

        while(fast != null && fast.next != null){
            if(slow == fast) return true;
            slow = slow.next;
            fast = fast.next.next;
        }
        return false;
    }

    public static Node addToFront(Node head, int data){
        Node newNode = new Node(data);
        newNode.next = head;
        head = newNode;
        return head;
    }

    public static void Main(String []args){
        Node head = null;
        head = addToFront(head, 1);
        head = addToFront(head, 2);
        head = addToFront(head, 3);
        head = addToFront(head, 4);
        Console.WriteLine(isCircular(head));
    }
}
// THIS CODE IS CONTRIBUTED BY KIRTI AGARWAL(KIRTIAGARWAL23121999)

Javascript

// JavaScript Program for the above approach
// structure of linked list node
class Node{
    constructor(data){
        this.data = data;
        this.next = null;
    }
}
let head = null;
function addToFront(data){
    let newNode = new Node(data);
    newNode.next = head;
    head = newNode;
}

function isCircular(){
    if(head == null) return false;
    let slow = head;
    let fast = head.next;
    while(fast != null && fast.next != null){
        if(slow == fast) return true;
        slow = slow.next;
        fast = fast.next.next;
    }
    return false;
}

// driver function
addToFront(1);
addToFront(2);
addToFront(3);
addToFront(4);

console.log(isCircular());

// THIS CODE IS CONTRIBUTED BY YASH AGARWAL(YASHAGARWAL2852002)

Output

false

Time Complexity – O(n) – We traverse the linked list in the worst case once, therefore, the time complexity here is linear.

Space Complexity – O(1) – We use two Node pointers, slow and fast, so the space complexity is constant.

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.