Circular vs Linear Linked List Key Difference in DSA Javascript - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a linear linked list node pointing to null.

DSA Javascript

const node = { value: 10, next: [1] };

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

Aundefined

Bnode

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the node itself as next creates a circular list.

Using undefined or 0 does not clearly mark the end.

In a linear linked list, the last node's next pointer is null to indicate the end of the list.

2fill in blank

medium

Complete the code to create a circular linked list node pointing to itself.

DSA Javascript

const node = { value: 20 }; node.next = [1];

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

A0

Bundefined

Cnull

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to null breaks the circular property.

Using undefined or 0 does not create a loop.

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

3fill in blank

hard

Fix the error in the function that checks if a linked list is circular.

DSA Javascript

function isCircular(head) {
  if (!head) return false;
  let current = head.next;
  while (current !== null && current !== head) {
    current = [1];
  }
  return current === head;
}

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

Acurrent.next

Bnull

Chead

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using head.next causes infinite loop or wrong traversal.

Assigning current to null ends the loop prematurely.

To traverse the list, we must move current to current.next each step.

4fill in blank

hard

Fill both blanks to complete the function that adds a node to the end of a circular linked list.

DSA Javascript

function addToCircularList(tail, value) {
  const newNode = { value: value, next: [1] };
  newNode.next = tail.next;
  tail.next = newNode;
  return [2];
}

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

Atail.next

BnewNode

Cnull

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to null breaks circularity.

Returning old tail instead of newNode loses the updated tail.

The new node's next points to the head (tail.next), and the new node becomes the new tail.

5fill in blank

hard

Fill all three blanks to complete the function that prints values of a circular linked list starting from head.

DSA Javascript

function printCircularList(head) {
  if (!head) return;
  let current = head;
  do {
    console.log(current.[1]);
    current = current.[2];
  } while (current !== [3]);
}

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

Avalue

Bnext

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev instead of next causes errors.

Comparing current to null breaks the loop in circular list.

We print current.value, move to current.next, and stop when we reach head again.

Practice

1. What is the main difference between a circular linked list and a linear linked list?

easy

A. In a circular linked list, the last node points back to the first node; in a linear linked list, the last node points to null.

B. In a linear linked list, nodes form a circle; in a circular linked list, nodes end with null.

C. Circular linked lists store data in a circle shape; linear linked lists store data in a straight line.

D. Circular linked lists use arrays internally; linear linked lists use pointers.

Solution

1. Step 1: Understand the pointer of the last node in linear linked list

   In a linear linked list, the last node's next pointer is null, indicating the end of the list.

2. Step 2: Understand the pointer of the last node in circular linked list

   In a circular linked list, the last node's next pointer points back to the first node, forming a circle.

3. Final Answer:

   In a circular linked list, the last node points back to the first node; in a linear linked list, the last node points to null. -> Option A

4. Quick Check:

   Last node pointer difference = In a circular linked list, the last node points back to the first node; in a linear linked list, the last node points to null. [OK]

Hint: Last node points null in linear, loops to head in circular [OK]

Common Mistakes:

• Confusing the direction of pointers in circular vs linear
• Thinking circular lists end with null
• Assuming circular lists use arrays internally

2. Which of the following is the correct way to check if a linked list is circular in JavaScript?

easy

A. if (head.next === head) { /* circular list */ } else if (head.next === null) { /* linear list */ }

B. if (head === null) { /* empty list */ } else if (head.next === head) { /* circular list */ }

C. if (head.next === null) { /* linear list */ } else { /* circular list */ }

D. if (head.next === undefined) { /* circular list */ }

Solution

1. Step 1: Check the next pointer of head node

   If head.next points to head itself, it means the list is circular with one node.

2. Step 2: Check if next pointer is null for linear list

   If head.next is null, it means the list ends there, so it is linear.

3. Final Answer:

   if (head.next === head) { /* circular list */ } else if (head.next === null) { /* linear list */ } -> Option A

4. Quick Check:

   Check next pointer for circular or null for linear [OK]

Hint: Check if next points to head for circular, null for linear [OK]

Common Mistakes:

• Checking undefined instead of null
• Assuming head.next null means circular
• Not handling empty list (head null) case

3. Consider the following JavaScript code for a circular linked list with three nodes. What will be the output when traversing and printing the list once starting from head?

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

const head = new Node(1);
const second = new Node(2);
const third = new Node(3);

head.next = second;
second.next = third;
third.next = head; // circular link

let current = head;
let output = '';
do {
  output += current.data + ' -> ';
  current = current.next;
} while (current !== head);
output += 'null';
console.log(output);

medium

A. Infinite loop, no output

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

C. 1 -> 2 -> 3 -> 1 -> null

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

Solution

1. Step 1: Understand the circular linked list structure

   The last node points back to the head, creating a loop: 1 -> 2 -> 3 -> 1 ...

2. Step 2: Trace the traversal loop once

   The do-while loop prints nodes starting at head and stops when it reaches head again after one full cycle, printing "1 -> 2 -> 3 -> null".

3. Final Answer:

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

4. Quick Check:

   Traverse once until back to head = 1 -> 2 -> 3 -> null [OK]

Hint: Traverse until current equals head again to print once [OK]

Common Mistakes:

• Thinking loop prints infinitely
• Appending extra nodes after stopping
• Confusing do-while with while loop

4. The following code attempts to detect if a linked list is circular. What is the bug?

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

medium

A. It does not handle empty list (head null) case.

B. It incorrectly returns true for linear lists.

C. It may cause infinite loop if list has a cycle not pointing to head.

D. It uses strict equality instead of loose equality.

Solution

1. Step 1: Analyze the loop condition and pointer movement

   The loop moves through nodes until current is null, checking if current.next equals head.

2. Step 2: Identify the problem with cycles not pointing to head

   If the list has a cycle that does not point back to head, the loop never reaches null and never finds current.next === head, causing an infinite loop.

3. Final Answer:

   It may cause infinite loop if list has a cycle not pointing to head. -> Option C

4. Quick Check:

   Cycle detection fails if cycle not at head [OK]

Hint: Cycle may not point to head, causing infinite loop [OK]

Common Mistakes:

• Assuming all cycles point to head
• Not checking for null head before loop
• Confusing strict and loose equality

5. You have a linked list where the last node points back to the third node (not the head). Which type of linked list is this, and what is the key difference from a linear linked list?

hard

A. It is a linear linked list; the key difference is the last node points to the third node.

B. It is a circular linked list; the key difference is the last node points to a node inside the list, not null.

C. It is a doubly linked list; the key difference is nodes have two pointers.

D. It is a circular linked list; the key difference is the last node points to the head only.

Solution

1. Step 1: Identify the type of linked list by last node pointer

   The last node points back to the third node, creating a loop inside the list, which defines a circular linked list.

2. Step 2: Compare with linear linked list ending

   In a linear linked list, the last node points to null, marking the end. Here, the last node points inside the list, so it is circular.

3. Final Answer:

   It is a circular linked list; the key difference is the last node points to a node inside the list, not null. -> Option B

4. Quick Check:

   Last node points inside list = circular list [OK]

Hint: Last node points inside list means circular, not linear [OK]

Common Mistakes:

• Thinking only head pointer defines circular list
• Confusing doubly linked list with circular list
• Assuming last node must point to head for circular