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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a linked list is circular by comparing the next pointer of the last node.

DSA Typescript

if (lastNode.next === [1]) {
  console.log('List is circular');
} else {
  console.log('List is linear');
}

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

Anull

Bundefined

Chead

DlastNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using null instead of head for circular check.

The last node's next points to the head in a circular linked list.

2fill in blank

medium

Complete the code to traverse a linear linked list until the end (null).

DSA Typescript

let current = head;
while (current !== [1]) {
  console.log(current.value);
  current = current.next;
}

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

Aundefined

Bnull

Chead

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined or head instead of null.

Traversal stops when current becomes null in a linear linked list.

3fill in blank

hard

Complete the code to traverse a linear linked list until the end.

DSA Typescript

let current = head;
while (current !== null && current.next !== [1]) {
  current = current.next;
}
console.log('Reached end');

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

Anull

Bhead

Cundefined

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for head instead of null.

In a linear list, the end is detected by current.next being null, not head.

4fill in blank

hard

Fill both blanks to create a function that detects if a linked list is circular by using two pointers.

DSA Typescript

function isCircular(head: Node | null): boolean {
  let slow = head;
  let fast = head;
  while (fast !== [1] && fast.next !== [2]) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) return true;
  }
  return false;
}

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

Anull

Bhead

Cundefined

Dslow

Attempts:

3 left

💡 Hint

Common Mistakes

Using head or undefined instead of null in conditions.

Both fast and fast.next must not be null to continue; null means end of linear list.

5fill in blank

hard

Fill all three blanks to create a function that returns the last node of a linear linked list.

DSA Typescript

function getLastNode(head: Node | null): Node | null {
  if (head === [1]) return null;
  let current = head;
  while (current.next !== [2]) {
    current = current.[3];
  }
  return current;
}

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

Anull

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using head instead of null for empty check.

Not moving current to next node.

Check if head is null first, then loop until current.next is null, moving current to current.next each time.

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, forming a circle.

B. In a linear linked list, the last node points back to the first node, forming a circle.

C. A circular linked list has no nodes.

D. A linear linked list always has only one node.

Solution

1. Step 1: Understand node connections in linear linked list

   In a linear linked list, nodes are connected one after another, and the last node points to null, ending the list.

2. Step 2: Understand node connections in circular linked list

   In a circular linked list, the last node points back to the first node, creating a loop or circle.

3. Final Answer:

   In a circular linked list, the last node points back to the first node, forming a circle. -> Option A

4. Quick Check:

   Circular list loops back = In a circular linked list, the last node points back to the first node, forming a circle. [OK]

Hint: Circular loops back to start; linear ends with null [OK]

Common Mistakes:

• Confusing which list loops back
• Thinking linear list forms a circle
• Assuming circular list has no end node

2. Which of the following is the correct way to check if a single-node linked list is circular in TypeScript?

easy

A. if (head.next === 0) { /* linear list */ }

B. if (head.next === null) { /* linear list */ }

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

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

Solution

1. Step 1: Understand node pointer in circular list

   In a circular linked list, the next pointer of the last node points back to the head node itself.

2. Step 2: Check the condition for circular list

   If head.next equals head, it means the list loops back to the start, confirming circularity.

3. Final Answer:

   if (head.next === head) { /* circular list */ } -> Option D

4. Quick Check:

   Next points to head means circular [OK]

Hint: Check if next points to head for circular list [OK]

Common Mistakes:

• Checking for null instead of head
• Using undefined instead of null
• Confusing zero with null pointer

3. Consider this TypeScript code for a circular linked list:

class Node {
  value: number;
  next: Node | null;
  constructor(value: number) {
    this.value = value;
    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;

let current = head;
let count = 0;
while (count < 5) {
  console.log(current.value);
  current = current.next!;
  count++;
}

What will be the output printed?

medium

A. 1 2 3 null null

B. 1 2 3 1 2

C. 1 2 3 4 5

D. 1 2 3 3 3

Solution

1. Step 1: Understand the circular linked list structure

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

2. Step 2: Trace the loop for 5 iterations

   Starting at head (1), the values printed are: 1, 2, 3, then loops back to 1, then 2.

3. Final Answer:

   1 2 3 1 2 -> Option B

4. Quick Check:

   Circular list repeats nodes in order [OK]

Hint: Circular list repeats nodes; count controls output length [OK]

Common Mistakes:

• Assuming list ends after 3 nodes
• Printing null after last node
• Confusing node values after loop

4. The following TypeScript code attempts to detect if a linked list is circular but has a bug:

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

What is the bug in this code?

medium

A. It incorrectly returns true for linear lists.

B. It does not handle empty lists (head = null).

C. It may enter an infinite loop if the list is circular but not pointing directly to head.

D. It uses 'current.next!' which causes a syntax error.

Solution

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

   The loop moves current to current.next until current is null or current.next equals head.

2. Step 2: Identify the problem with circular lists

   If the list is circular but the loop does not point directly back to head (e.g., cycle in middle), the loop never ends, causing infinite loop.

3. Final Answer:

   It may enter an infinite loop if the list is circular but not pointing directly to head. -> Option C

4. Quick Check:

   Loop may never end if cycle not at head [OK]

Hint: Check for cycles anywhere, not just pointing to head [OK]

Common Mistakes:

• Assuming only last node points to head
• Ignoring infinite loop possibility
• Confusing syntax error with logic error

5. You want to convert a linear linked list into a circular linked list in TypeScript. Given the code below, which line correctly completes the conversion?

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

const head = new Node(10);
const second = new Node(20);
const third = new Node(30);

head.next = second;
second.next = third;
third.next = null; // currently linear list

// Which line below makes the list circular?
// A: third.next = null;
// B: second.next = head;
// C: head.next = third;
// D: third.next = head;

medium

A. third.next = head;

B. second.next = head;

C. head.next = third;

D. third.next = null;

Solution

1. Step 1: Understand the current list structure

   The list is linear: head -> second -> third -> null.

2. Step 2: Make the last node point to head to form a circle

   Setting third.next = head connects the last node back to the first, making the list circular.

3. Final Answer:

   third.next = head; -> Option A

4. Quick Check:

   Last node points to head for circular list [OK]

Hint: Last node's next must point to head for circular list [OK]

Common Mistakes:

• Setting next to null keeps list linear
• Pointing head.next to third breaks order
• Pointing second.next to head creates wrong cycle