Create a Circular Singly Linked List in DSA Javascript - Build from Scratch

Create a Circular Singly Linked List

📖 Scenario: You are building a simple circular singly linked list to manage a playlist of songs that loops back to the start after the last song.

🎯 Goal: Create a circular singly linked list with three nodes containing song names, link them in a circle, and print the list to show the circular structure.

📋 What You'll Learn

Create a Node class with value and next properties

Create three nodes with values 'Song1', 'Song2', and 'Song3'

Link the nodes so that the last node points back to the first node, forming a circle

Print the circular linked list starting from the head node, showing all nodes once

💡 Why This Matters

🌍 Real World

Circular linked lists are used in applications like music playlists, round-robin scheduling, and buffering where the list loops continuously.

💼 Career

Understanding circular linked lists helps in roles involving system design, game development, and real-time data processing where cyclic data structures are common.

1

Create Node class and three nodes

Create a class called Node with a constructor that takes value and sets this.value and this.next to null. Then create three nodes called node1, node2, and node3 with values 'Song1', 'Song2', and 'Song3' respectively.

DSA Javascript

# Your code here

Hint

Define the Node class first, then create three instances with the exact names and values.

2

Link nodes to form a circular list

Link the nodes by setting node1.next to node2, node2.next to node3, and node3.next to node1 to form a circle.

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

const node1 = new Node('Song1');
const node2 = new Node('Song2');
const node3 = new Node('Song3');

# Your code here

Hint

Set the next property of each node to the next node, and the last node back to the first.

3

Create a function to print the circular list

Create a function called printCircularList that takes a head node and prints the values of the nodes in the circular list once, separated by -> . Use a do-while loop to traverse until you reach the head again.

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

const node1 = new Node('Song1');
const node2 = new Node('Song2');
const node3 = new Node('Song3');

node1.next = node2;
node2.next = node3;
node3.next = node1;

# Your code here

Hint

Use a do-while loop to print each node's value until you circle back to the head.

4

Print the circular linked list

Call the function printCircularList with node1 as the argument to print the circular linked list.

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

const node1 = new Node('Song1');
const node2 = new Node('Song2');
const node3 = new Node('Song3');

node1.next = node2;
node2.next = node3;
node3.next = node1;

function printCircularList(head) {
  let current = head;
  let result = '';
  do {
    result += current.value + ' -> ';
    current = current.next;
  } while (current !== head);
  result += 'null';
  console.log(result);
}

# Your code here

Hint

Call printCircularList(node1) to see the circular list printed.

Practice

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

easy

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

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

C. It stores data in a sorted order automatically.

D. It uses an array internally to store nodes.

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, circular singly linked lists have only one pointer per node and form a circle, not a linear chain.

3. Final Answer:

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

4. Quick Check:

   Last node's next = first node [OK]

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

Common Mistakes:

• Confusing circular singly linked list with doubly linked list
• Thinking nodes have two pointers
• Assuming data is sorted automatically

2. Which of the following is the correct way to initialize an empty circular singly linked list in JavaScript?

easy

A. const head = { data: null, next: head };

B. const head = null; // no nodes yet

C. const head = { data: null, next: null };

D. const head = []; // empty array

Solution

1. Step 1: Understand empty list initialization

   An empty circular singly linked list has no nodes, so the head pointer is null.

2. Step 2: Check other options

   const head = { data: null, next: null }; creates a node with null data and next pointer, which is not empty. const head = { data: null, next: head }; references head before assignment, causing error. const head = []; // empty array uses an array, not a linked list.

3. Final Answer:

   const head = null; // no nodes yet -> Option B

4. Quick Check:

   Empty list head = null [OK]

Hint: Empty list means head is null, no nodes created [OK]

Common Mistakes:

• Creating a node with null data instead of null head
• Referencing head before declaration
• Using arrays instead of linked list nodes

3. What will be the output of the following code snippet that creates a circular singly linked list with nodes 1, 2, 3 and prints the list once?

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

class CircularSinglyLinkedList {
  constructor() {
    this.head = null;
  }

  append(data) {
    const newNode = new Node(data);
    if (!this.head) {
      this.head = newNode;
      newNode.next = this.head;
    } else {
      let temp = this.head;
      while (temp.next !== this.head) {
        temp = temp.next;
      }
      temp.next = newNode;
      newNode.next = this.head;
    }
  }

  printList() {
    if (!this.head) return '';
    let result = '';
    let temp = this.head;
    do {
      result += temp.data + ' -> ';
      temp = temp.next;
    } while (temp !== this.head);
    result += 'null';
    return result;
  }
}

const list = new CircularSinglyLinkedList();
list.append(1);
list.append(2);
list.append(3);
console.log(list.printList());

medium

A. null

B. 1 -> 2 -> 3 -> 1 -> 2 -> 3 -> null

C. 1 -> 2 -> 3

D. 1 -> 2 -> 3 -> null

Solution

1. Step 1: Understand append method behavior

   Each new node is added at the end, and its next points to head, maintaining circularity.

2. Step 2: Trace printList method

   printList starts at head and loops until it reaches head again, appending data with ' -> '. Finally adds 'null'. So output is '1 -> 2 -> 3 -> null'.

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option D

4. Quick Check:

   Print once around circle = 1 -> 2 -> 3 -> null [OK]

Hint: Print loops until back to head once, then stops [OK]

Common Mistakes:

• Printing infinite loop by not stopping at head
• Missing 'null' at end
• Printing nodes multiple times

4. Identify the bug in this code snippet that tries to append a node to a circular singly linked list:

append(data) {
  const newNode = new Node(data);
  if (!this.head) {
    this.head = newNode;
    newNode.next = null;
  } else {
    let temp = this.head;
    while (temp.next !== this.head) {
      temp = temp.next;
    }
    temp.next = newNode;
    newNode.next = this.head;
  }
}

medium

A. newNode.next should point to this.head when list is empty, not null.

B. The while loop condition should be temp.next === this.head.

C. The head should be set to newNode every time a node is appended.

D. The newNode should not be created inside the append method.

Solution

1. Step 1: Check initialization for empty list

   When list is empty, newNode.next must point to head itself to maintain circularity, not null.

2. Step 2: Verify loop and pointer updates

   The while loop and pointer updates are correct; only the initial next pointer is wrong.

3. Final Answer:

   newNode.next should point to this.head when list is empty, not null. -> Option A

4. Quick Check:

   Empty list node next = head, not null [OK]

Hint: Empty list node next must loop to itself [OK]

Common Mistakes:

• Setting newNode.next to null breaks circular link
• Wrong while loop condition
• Resetting head incorrectly

5. You want to insert a new node with value 4 after the node with value 3 in a circular singly linked list. Given the list: 3 -> 1 -> 2 -> (back to 3), which code snippet correctly performs this insertion?

hard

A. let temp = head;
while (temp.next !== head) {
temp = temp.next;
}
const newNode = new Node(4);
temp.next = newNode;
newNode.next = head;

B. const newNode = new Node(4);
newNode.next = head;
head.next = newNode;

C. let temp = head;
while (temp.data !== 3) {
temp = temp.next;
}
const newNode = new Node(4);
newNode.next = temp.next;
temp.next = newNode;

D. head = new Node(4);
head.next = oldHead;

Solution

1. Step 1: Locate node with value 3

   Traverse from head until temp.data equals 3 to find the correct insertion point.

2. Step 2: Insert new node after node 3

   Create newNode with data 4, set newNode.next to temp.next, then set temp.next to newNode to insert correctly.

3. Final Answer:

   Traverse from head until finding node with value 3, then insert new node after it. -> Option C

4. Quick Check:

   Insert after node 3 by adjusting pointers [OK]

Hint: Find node 3, insert new node by pointer swap [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating pointers correctly
• Replacing head incorrectly