Create a Circular Singly Linked List in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a node with a value and next pointer.

DSA Javascript

class Node {
  constructor(value) {
    this.value = value;
    this.next = [1];
  }
}

Drag options to blanks, or click blank then click option'

A0

Bundefined

Cnull

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to undefined or 0 instead of null.

The next pointer should initially point to null because the node does not link to any other node yet.

2fill in blank

medium

Complete the code to initialize the head of the circular linked list pointing to itself.

DSA Javascript

class CircularLinkedList {
  constructor() {
    this.head = new Node(10);
    this.head.next = [1];
  }
}

Drag options to blanks, or click blank then click option'

Athis.head

Bnew Node(10)

Cundefined

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to null breaks the circular structure.

In a circular singly linked list with one node, the next pointer points back to the head node itself.

3fill in blank

hard

Fix the error in the insert method to correctly insert a new node after the head.

DSA Javascript

insert(value) {
  const newNode = new Node(value);
  newNode.next = this.head.next;
  this.head.next = [1];
}

Drag options to blanks, or click blank then click option'

AnewNode

Bnew Node(value)

Cvalue

Dthis.head

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning head.next to head or value instead of newNode.

The head's next pointer should point to the new node to insert it after the head.

4fill in blank

hard

Fill both blanks to complete the method that prints all values in the circular linked list.

DSA Javascript

printList() {
  if (!this.head) return;
  let current = this.head;
  do {
    console.log(current.value);
    current = [1];
  } while (current !== [2]);
}

Drag options to blanks, or click blank then click option'

Acurrent.next

Bthis.head

Cthis.head.next

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Using current instead of current.next causes infinite loop.

Comparing to current instead of head stops too early.

We move current to current.next each time and stop when current reaches the head again.

5fill in blank

hard

Fill all three blanks to complete the method that searches for a value in the circular linked list.

DSA Javascript

search(value) {
  if (!this.head) return false;
  let current = this.head;
  do {
    if (current.[1] === [2]) return true;
    current = [3];
  } while (current !== this.head);
  return false;
}

Drag options to blanks, or click blank then click option'

Avalue

Ccurrent.next

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'current.next' causes errors.

Comparing wrong properties or variables.

We check if current.value equals the searched value and move to current.next each time.

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