Create a Circular Singly Linked List in DSA C++

Mental Model

A circular singly linked list is like a chain where the last link connects back to the first, forming a circle.

Analogy: Imagine a round table where people sit in a circle; each person points to the next person, and the last person points back to the first, so you can keep going around forever.

head -> 1 -> 2 -> 3 ->
↑                   ↓
←←←←←←←←←←←←←←←←←←←

Dry Run Walkthrough

Input: Create circular singly linked list with values 1, 2, 3 in order

Goal: Build a circular singly linked list where last node points back to head

Step 1: Create node with value 1 and point head to it

head -> [1] -> null

Why: Start the list with the first node

Step 2: Create node with value 2 and link first node to it

head -> 1 -> [2] -> null

Why: Add second node after first

Step 3: Create node with value 3 and link second node to it

head -> 1 -> 2 -> [3] -> null

Why: Add third node after second

Step 4: Link last node (3) back to head (1) to form circle

head -> 1 -> 2 -> 3 -> [head]

Why: Close the list into a circle by linking last to first

Result:

head -> 1 -> 2 -> 3 -> head (circular)

Annotated Code

DSA C++

#include <iostream>
using namespace std;

class Node {
public:
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

class CircularSinglyLinkedList {
public:
    Node* head;
    CircularSinglyLinkedList() : head(nullptr) {}

    void createFromArray(int arr[], int n) {
        if (n == 0) return; // empty array, no list
        head = new Node(arr[0]);
        Node* curr = head;
        for (int i = 1; i < n; i++) {
            curr->next = new Node(arr[i]);
            curr = curr->next; // advance curr to new node
        }
        curr->next = head; // link last node back to head to form circle
    }

    void printList(int n) {
        Node* curr = head;
        for (int i = 0; i < n; i++) {
            cout << curr->data;
            if (i != n - 1) cout << " -> ";
            curr = curr->next; // advance curr to next node
        }
        cout << " -> head" << endl;
    }
};

int main() {
    CircularSinglyLinkedList list;
    int arr[] = {1, 2, 3};
    list.createFromArray(arr, 3);
    list.printList(3);
    return 0;
}

head = new Node(arr[0]);

create first node and assign head

curr->next = new Node(arr[i]);

link new node after current

curr = curr->next;

advance curr to newly created node

curr->next = head;

link last node back to head to form circle

curr = curr->next;

advance curr to next node during print

Output

1 -> 2 -> 3 -> head

Complexity Analysis

Time: O(n) because we create and link each of the n nodes once

Space: O(n) because we store n nodes in memory

vs Alternative: Compared to a linear singly linked list, circular list adds one extra link to connect last node to head, no extra traversal cost

Edge Cases

empty array input

no nodes created, head remains nullptr

DSA C++

if (n == 0) return;

single element array

one node created, next points to itself forming a single-node circle

DSA C++

curr->next = head;

When to Use This Pattern

When you need a list that loops back to start for repeated cycling, use circular singly linked list creation pattern.

Common Mistakes

Mistake: Forgetting to link the last node back to head, leaving list linear
Fix: Add curr->next = head; after creating all nodes to close the circle

Mistake: Not handling empty input array, causing null pointer errors
Fix: Check if input size is zero and return early

Summary

Creates a circular singly linked list where last node points back to first.

Use when you want to cycle through elements repeatedly without end.

The key is linking the last node's next pointer back to the head node.

Practice

1. What is a key characteristic of a circular singly linked list compared to a regular singly linked list?

easy

A. It uses an array internally to store nodes.

B. Each node has two pointers, one to the next and one to the previous node.

C. It cannot be traversed starting from the head node.

D. The last node points back to the first node, forming a circle.

Solution

1. Step 1: Understand the structure of a circular singly linked list

   In a circular singly linked list, the last node's next pointer points back to the first node, creating a loop.

2. Step 2: Compare with other linked lists

   Unlike doubly linked lists (which have two pointers) or arrays, circular singly linked lists have a single pointer per node and form a circle.

3. Final Answer:

   The last node points back to the first node, forming a circle. -> Option D

4. Quick Check:

   Circular list = last node points to first node [OK]

Hint: Remember: circular means last node links back to first [OK]

Common Mistakes:

• Thinking circular means two pointers per node
• Confusing with doubly linked list
• Assuming it uses arrays internally

2. Which of the following is the correct way to declare a node structure for a circular singly linked list in C++?

easy

A. struct Node { int data; int next; }; // Incorrect pointer type

B. struct Node { int data; Node* prev; Node* next; }; // Doubly linked list node

C. struct Node { int data; Node* next; }; // Correct declaration

D. struct Node { int data; Node next; }; // Incorrect pointer usage

Solution

1. Step 1: Identify the correct node structure for singly linked list

   A singly linked list node has data and a pointer to the next node. The pointer type must be Node*.

2. Step 2: Check each option

   struct Node { int data; Node* next; }; // Correct declaration correctly declares Node* next. struct Node { int data; Node* prev; Node* next; }; // Doubly linked list node has an extra prev pointer (doubly linked list). struct Node { int data; int next; }; // Incorrect pointer type uses int next which is wrong type. struct Node { int data; Node next; }; // Incorrect pointer usage uses Node next which is not a pointer.

3. Final Answer:

   struct Node { int data; Node* next; }; -> Option C

4. Quick Check:

   Single pointer to next node = correct node structure [OK]

Hint: Node must have pointer to next node only [OK]

Common Mistakes:

• Using non-pointer type for next
• Adding prev pointer for singly linked list
• Declaring next as object instead of pointer

3. What will be the output of the following code snippet that creates a circular singly linked list with 3 nodes and prints their data?

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

int main() {
    Node* head = new Node{1, nullptr};
    Node* second = new Node{2, nullptr};
    Node* third = new Node{3, nullptr};

    head->next = second;
    second->next = third;
    third->next = head; // circular link

    Node* temp = head;
    for (int i = 0; i < 5; i++) {
        std::cout << temp->data << " ";
        temp = temp->next;
    }
    return 0;
}

medium

A. 1 2 3 1 2

B. 1 2 3 3 3

C. 1 2 3 4 5

D. 1 2 3 1 1

Solution

1. Step 1: Understand the circular link setup

   The last node (third) points back to head, so the list loops: 1 -> 2 -> 3 -> 1 -> ...

2. Step 2: Trace the print loop for 5 iterations

   Starting at head (1), print 1, move to 2, print 2, move to 3, print 3, move to 1, print 1, move to 2, print 2.

3. Final Answer:

   1 2 3 1 2 -> Option A

4. Quick Check:

   Loop prints nodes in circle: 1 2 3 1 2 [OK]

Hint: Circular list repeats nodes after last one [OK]

Common Mistakes:

• Assuming list ends after 3 nodes
• Printing wrong data due to pointer errors
• Confusing circular with linear list output

4. Identify the error in the following code that attempts to create a circular singly linked list with one node:

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

int main() {
    Node* head = new Node{10, nullptr};
    head->next = nullptr; // Intended to make circular
    return 0;
}

medium

A. Memory leak due to missing delete statement.

B. head->next should point to head, not nullptr, to form a circular list.

C. Using nullptr for next is correct for circular list.

D. Node declaration is missing a constructor.

Solution

1. Step 1: Check how circular link is formed

   For a circular singly linked list with one node, the node's next pointer must point to itself.

2. Step 2: Identify the mistake in the code

   The code sets head->next = nullptr, which breaks the circular property.

3. Final Answer:

   head->next should point to head, not nullptr, to form a circular list. -> Option B

4. Quick Check:

   One node circular list: next points to itself [OK]

Hint: One node circular list points next to itself [OK]

Common Mistakes:

• Setting next to nullptr instead of self
• Confusing circular with linear list
• Ignoring memory management issues (not relevant here)

5. You want to insert a new node with value 4 at the end of an existing circular singly linked list with nodes 1 -> 2 -> 3 -> back to 1. Which of the following code snippets correctly performs this insertion?

hard

A. Node* newNode = new Node{4, head};
Node* temp = head;
while (temp->next != head) temp = temp->next;
temp->next = newNode;

B. Node* newNode = new Node{4, nullptr};
Node* temp = head;
while (temp->next != nullptr) temp = temp->next;
temp->next = newNode;
newNode->next = head;

C. Node* newNode = new Node{4, head};
head->next = newNode;

D. Node* newNode = new Node{4, nullptr};
newNode->next = head;
head = newNode;

Solution

1. Step 1: Understand insertion at end in circular singly linked list

   To insert at end, find last node (whose next points to head), then link new node to head and last node to new node.

2. Step 2: Analyze each option

   Node* newNode = new Node{4, head}; Node* temp = head; while (temp->next != head) temp = temp->next; temp->next = newNode; correctly creates new node pointing to head, finds last node by looping until temp->next == head, then sets last node's next to new node. Node* newNode = new Node{4, nullptr}; Node* temp = head; while (temp->next != nullptr) temp = temp->next; temp->next = newNode; newNode->next = head; incorrectly checks for nullptr which does not exist in circular list. Node* newNode = new Node{4, head}; head->next = newNode; incorrectly sets head->next to new node, breaking list. Node* newNode = new Node{4, nullptr}; newNode->next = head; head = newNode; changes head pointer incorrectly and does not update last node's next.

3. Final Answer:

   Node* newNode = new Node{4, head}; Node* temp = head; while (temp->next != head) temp = temp->next; temp->next = newNode; -> Option A

4. Quick Check:

   Find last node by next == head, link new node to head, last node to new node [OK]

Hint: Loop until next points to head to find last node [OK]

Common Mistakes:

• Checking for nullptr in circular list
• Not updating last node's next pointer
• Incorrectly changing head pointer