Stack vs Array Direct Use Why We Need Stack Abstraction in DSA Typescript - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack using array methods.

DSA Typescript

const stack: number[] = [];
stack.[1](10);

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

Aunshift

Bshift

Cpush

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push removes elements instead of adding.

Using shift or unshift changes the start of the array, not the end.

The push method adds an element to the end of the array, which simulates pushing onto a stack.

2fill in blank

medium

Complete the code to remove the top element from the stack using array methods.

DSA Typescript

const stack: number[] = [1, 2, 3];
const top = stack.[1]();

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

Apush

Bshift

Cunshift

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop adds elements instead of removing.

Using shift removes the first element, not the last.

The pop method removes and returns the last element of the array, simulating the stack's pop operation.

3fill in blank

hard

Fix the error in the code that tries to access the top element of the stack without removing it.

DSA Typescript

const stack: number[] = [5, 10, 15];
const top = stack[[1]];

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

Astack.length - 1

Bstack.length

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack.length causes undefined because it's out of bounds.

Using 0 accesses the bottom of the stack, not the top.

The top element of the stack is at the last index, which is stack.length - 1.

4fill in blank

hard

Fill both blanks to implement a stack class method that pushes and then pops an element.

DSA Typescript

class Stack {
  private items: number[] = [];
  push(item: number) {
    this.items.[1](item);
  }
  pop() {
    return this.items.[2]();
  }
}

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

Apush

Bpop

Cshift

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift or unshift changes the start of the array, not the end.

Mixing push and shift or pop and unshift breaks stack order.

The push method adds an item to the stack, and pop removes the last item, following stack behavior.

5fill in blank

hard

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

DSA Typescript

class Stack {
  private items: number[] = [];
  peek() {
    if (this.items.length === 0) return undefined;
    return this.items[[1]];
  }
  size() {
    return this.items.[2];
  }
  isEmpty() {
    return this.items.[3] === 0;
  }
}

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

Athis.items.length - 1

Blength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 index for peek returns bottom element.

Using methods instead of properties for length causes errors.

The peek method accesses the last element at index length - 1. The size method returns the array's length. The isEmpty method checks if length is zero.

Practice

1. Why do we use a stack abstraction instead of directly using an array for stack operations?

easy

A. To restrict operations to only push and pop, preventing misuse

B. Because arrays cannot store multiple elements

C. To improve the speed of accessing elements randomly

D. Because stacks use less memory than arrays

Solution

1. Step 1: Understand stack operations

   A stack allows only push (add) and pop (remove) operations at one end, the top.

2. Step 2: Compare with array usage

   Arrays allow many operations like random access and insertion anywhere, which can cause misuse if used as stacks directly.

3. Final Answer:

   To restrict operations to only push and pop, preventing misuse -> Option A

4. Quick Check:

   Stack abstraction limits operations = A [OK]

Hint: Stack abstraction limits operations to push/pop only [OK]

Common Mistakes:

• Thinking arrays cannot store multiple elements
• Believing stacks improve random access speed
• Assuming stacks always use less memory

2. Which of the following TypeScript code snippets correctly implements a stack push operation using an array?

easy

A. stack.add(element);

B. stack.pop(element);

C. stack.insert(element);

D. stack.push(element);

Solution

1. Step 1: Identify correct array method for push

   In TypeScript, arrays use the push() method to add elements at the end.

2. Step 2: Check other options

   pop() removes elements, insert() and add() are not valid array methods.

3. Final Answer:

   stack.push(element); -> Option D

4. Quick Check:

   Array push method = push() [OK]

Hint: Use push() to add elements to array end [OK]

Common Mistakes:

• Using pop() to add elements
• Using non-existent methods like insert() or add()
• Confusing push() with pop()

3. What will be the output of the following TypeScript code?

const stack: number[] = [];
stack.push(10);
stack.push(20);
const popped = stack.pop();
console.log(stack);
console.log(popped);

medium

A. [10]\n20

B. [20]\n10

C. [10, 20]\n20

D. []\n10

Solution

1. Step 1: Trace push operations

   stack.push(10) adds 10, stack.push(20) adds 20, so stack is [10, 20].

2. Step 2: Trace pop operation

   stack.pop() removes and returns the last element 20, so popped = 20 and stack becomes [10].

3. Final Answer:

   [10]\n20 -> Option A

4. Quick Check:

   Pop removes last element = [10] and popped = 20 [OK]

Hint: pop() removes last pushed element from array [OK]

Common Mistakes:

• Assuming pop() removes first element
• Confusing popped value with stack content
• Expecting stack unchanged after pop()

4. Identify the error in this TypeScript stack implementation using an array:

class Stack {
  private items: number[] = [];
  push(item: number) {
    this.items.add(item);
  }
  pop() {
    return this.items.pop();
  }
}

medium

A. Missing return type for pop() method

B. Using add() method which does not exist on arrays

C. items array should be public, not private

D. pop() should remove first element, not last

Solution

1. Step 1: Check push method implementation

   The push method uses this.items.add(item), but arrays do not have an add() method.

2. Step 2: Verify correct method for adding

   The correct method to add an element to an array is push(), so it should be this.items.push(item).

3. Final Answer:

   Using add() method which does not exist on arrays -> Option B

4. Quick Check:

   Array add() method invalid, use push() [OK]

Hint: Array method to add is push(), not add() [OK]

Common Mistakes:

• Thinking add() is valid array method
• Ignoring method name errors
• Confusing pop() behavior

5. You want to ensure that users of your stack cannot access elements except through push and pop. Which approach best enforces this in TypeScript?

hard

A. Use an array and allow direct access to elements

B. Use a public array and document to use only push and pop

C. Use a class with a private array and expose only push and pop methods

D. Use a global array variable and functions to push and pop

Solution

1. Step 1: Understand encapsulation in TypeScript

   Making the array private inside a class hides it from outside access.

2. Step 2: Provide controlled access

   Exposing only push and pop methods allows controlled stack operations, preventing misuse.

3. Final Answer:

   Use a class with a private array and expose only push and pop methods -> Option C

4. Quick Check:

   Private array + limited methods = safe stack [OK]

Hint: Use private array and expose only push/pop methods [OK]

Common Mistakes:

• Making array public and trusting users
• Using global variables without encapsulation
• Allowing direct element access