Stack Implementation Using Linked List in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Typescript

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

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

Aundefined

B0

Cthis

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to undefined or 0 instead of null.

When creating a new node, its next pointer should start as null because it doesn't point to any other node yet.

2fill in blank

medium

Complete the push method to add a new node at the top of the stack.

DSA Typescript

push(value: number): void {
  const newNode = new Node(value);
  newNode.next = this.top;
  this.top = [1];
  this.size++;
}

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

AnewNode

Bthis.top

Cthis.size

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning top to value or size instead of the new node.

To add a new node on top, we set the stack's top to the new node.

3fill in blank

hard

Fix the error in the pop method to correctly remove and return the top node's value.

DSA Typescript

pop(): number | null {
  if (!this.top) return null;
  const poppedNode = this.top;
  this.top = poppedNode.next;
  this.size--;
  return [1];
}

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

ApoppedNode.value

Bthis.top.value

CpoppedNode

Dthis.size

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the node object or the new top's value instead of the popped node's value.

We return the value of the popped node, not the node itself or other properties.

4fill in blank

hard

Fill both blanks to check if the stack is empty and return a boolean.

DSA Typescript

isEmpty(): boolean {
  return this.size [1] 0;
}

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

A!==

B===

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or !== which give wrong boolean results for emptiness.

The stack is empty if its size is exactly zero.

5fill in blank

hard

Fill all three blanks to implement peek method that returns the top value without removing it.

DSA Typescript

peek(): number | null {
  if (this.top === [1]) return null;
  return this.top[2][3];
}

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

Anull

B.

Cvalue

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Returning undefined or accessing value without checking if top is null.

If top is null, return null. Otherwise, return top.value using dot notation.

Practice

1. What is the main advantage of implementing a stack using a linked list instead of an array?

easy

A. It allows random access to elements

B. It uses less memory than an array

C. The stack can grow dynamically without a fixed size limit

D. It is faster to access elements by index

Solution

1. Step 1: Understand stack size limitation in arrays

   Arrays have a fixed size, so stacks implemented with arrays have a maximum capacity.

2. Step 2: Linked list dynamic size

   Linked lists can grow or shrink dynamically by adding or removing nodes, so stack size is not fixed.

3. Final Answer:

   The stack can grow dynamically without a fixed size limit -> Option C

4. Quick Check:

   Linked list stacks grow dynamically = A [OK]

Hint: Linked list stacks grow dynamically, arrays do not [OK]

Common Mistakes:

• Thinking linked lists use less memory always
• Confusing stack with queue behavior
• Assuming random access is possible in linked lists

2. Which of the following is the correct TypeScript syntax to define a node class for a linked list stack?

easy

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

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

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

D. class Node { value: number; next: any; constructor(value: number) { this.value = value; this.next = undefined; } }

Solution

1. Step 1: Check next pointer type

   In a linked list, next can be another node or null if it's the last node.

2. Step 2: Verify constructor initialization

   The constructor sets value and initializes next to null.

3. Final Answer:

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

4. Quick Check:

   next can be Node or null = B [OK]

Hint: next must allow null for last node [OK]

Common Mistakes:

• Not allowing next to be null
• Initializing next as undefined
• Using incorrect type for next

3. Given the following TypeScript code for a stack push operation, what will be the top node's value after pushing 10, then 20?

medium

A. undefined

B. 10

C. null

D. 20

Solution

1. Step 1: Understand push operation

   Push adds a new node at the top, so the last pushed value becomes the top.

2. Step 2: Trace pushing 10 then 20

   First push 10: top.value = 10; then push 20: top.value = 20 (new node).

3. Final Answer:

   20 -> Option D

4. Quick Check:

   Last pushed value is top = 20 [OK]

Hint: Top always holds last pushed value [OK]

Common Mistakes:

• Assuming bottom value is top
• Confusing push with pop
• Not updating top pointer

4. Identify the error in this pop method of a linked list stack implementation:

pop(): number | null {
  if (!this.top) return null;
  const value = this.top.value;
  this.top = this.top.next;
  return this.top.value;
}

medium

A. Returns the new top value instead of popped value

B. Does not check if stack is empty

C. Does not update top pointer

D. Throws error when stack is empty

Solution

1. Step 1: Analyze return value

   The method returns this.top.value after updating top, which is the new top, not the popped value.

2. Step 2: Correct return value

   It should return the saved value before updating top to return the popped element.

3. Final Answer:

   Returns the new top value instead of popped value -> Option A

4. Quick Check:

   Pop must return popped value, not new top [OK]

Hint: Return saved value before updating top [OK]

Common Mistakes:

• Returning top after update instead of popped value
• Not handling empty stack
• Forgetting to update top pointer

5. You want to implement a method peek() in your linked list stack that returns the top value without removing it. Which is the correct implementation?

hard

A. peek(): number | null { return this.top ? this.top.value : null; }

B. peek(): number | null { const val = this.top.value; this.top = this.top.next; return val; }

C. peek(): number | null { if (!this.top) return null; this.top = this.top.next; return this.top.value; }

D. peek(): number | null { return this.top.next ? this.top.next.value : null; }

Solution

1. Step 1: Understand peek operation

   Peek returns the top value without changing the stack.

2. Step 2: Check each option

   peek(): number | null { return this.top ? this.top.value : null; } returns top value if exists, else null, without modifying stack. Others modify top or return wrong node.

3. Final Answer:

   peek(): number | null { return this.top ? this.top.value : null; } -> Option A

4. Quick Check:

   Peek returns top value without removal = C [OK]

Hint: Peek returns top value without changing stack [OK]

Common Mistakes:

• Modifying top pointer in peek
• Returning next node's value
• Not handling empty stack