Stack Implementation Using Array in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the stack.

DSA Typescript

push(element: number): void {
  this.items.[1](element);
}

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

Aappend

Bpush

Cadd

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' which is not a valid array method in TypeScript.

The push method adds an element to the end of the array representing the stack.

2fill in blank

medium

Complete the code to remove the top element from the stack.

DSA Typescript

pop(): number | undefined {
  return this.items.[1]();
}

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

Apop

Bdelete

Cshift

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'shift' which removes the first element, not the last.

The pop method removes and returns the last element of the array, which is the top of the stack.

3fill in blank

hard

Fix the error in the method that returns the top element without removing it.

DSA Typescript

peek(): number | undefined {
  return this.items[this.items.[1] - 1];
}

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

Acount

Bsize

Clength

Dlength()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length()' which is not a valid property but a function call.

The length property gives the number of elements in the array. We subtract 1 to get the last index.

4fill in blank

hard

Fill both blanks to check if the stack is empty.

DSA Typescript

isEmpty(): boolean {
  return this.items.[1] === [2];
}

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

Alength

B0

Cnull

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing length to null or -1 which are incorrect.

The stack is empty if the array length is zero.

5fill in blank

hard

Fill all three blanks to clear the stack.

DSA Typescript

clear(): void {
  this.items = [1];
  this.size = [2];
  this.top = [3];
}

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

A[]

B0

C-1

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting size or top to null instead of numbers.

To clear the stack, reset the items to an empty array, size to zero, and top index to -1.

Practice

1. What is the main characteristic of a stack data structure implemented using an array?

easy

A. It stores elements in sorted order

B. It follows First In First Out (FIFO) order

C. It allows random access to elements

D. It follows Last In First Out (LIFO) order

Solution

1. Step 1: Understand stack behavior

   A stack adds and removes elements from the same end, so the last element added is the first to be removed.

2. Step 2: Identify the order type

   This behavior is called Last In First Out (LIFO).

3. Final Answer:

   It follows Last In First Out (LIFO) order -> Option D

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack always removes the last added item first [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking stack sorts elements
• Assuming random access like arrays

2. Which of the following is the correct way to push an element 'x' onto a stack implemented as an array named stack in TypeScript?

easy

A. stack.insert(x);

B. stack.pop(x);

C. stack.push(x);

D. stack.add(x);

Solution

1. Step 1: Recall array methods for stack push

   In TypeScript, to add an element at the end of an array, use the push method.

2. Step 2: Verify other options

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

3. Final Answer:

   stack.push(x); -> Option C

4. Quick Check:

   Push uses .push() method [OK]

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

Common Mistakes:

• Using pop instead of push
• Using non-existent methods like insert or add
• Confusing push with pop

3. What will be the output of the following TypeScript code implementing a stack using an array?

const stack: number[] = [];
stack.push(10);
stack.push(20);
stack.pop();
stack.push(30);
console.log(stack);

medium

A. [10, 20, 30]

B. [10, 30]

C. [20, 30]

D. [30]

Solution

1. Step 1: Trace stack operations step-by-step

   Initially stack = []. After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top element 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine final stack state

   The console.log prints the current stack array: [10, 30].

3. Final Answer:

   [10, 30] -> Option B

4. Quick Check:

   Push and pop update stack correctly [OK]

Hint: Pop removes last pushed element, then push adds new [OK]

Common Mistakes:

• Forgetting pop removes last element
• Assuming pop removes first element
• Confusing order of push and pop

4. Identify the error in the following TypeScript code for stack pop operation and select the correct fix.

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

medium

A. pop() should check if stack is empty before popping

B. items array should be public, not private

C. pop() should return void, not number

D. No error, code is correct

Solution

1. Step 1: Analyze pop method behavior

   The pop() method of array returns number | undefined if empty. Returning directly without check can cause undefined return.

2. Step 2: Identify fix for empty stack

   To avoid returning undefined, pop() should check if items is empty before popping and handle that case.

3. Final Answer:

   pop() should check if stack is empty before popping -> Option A

4. Quick Check:

   Check empty before pop to avoid undefined [OK]

Hint: Always check stack empty before pop to avoid errors [OK]

Common Mistakes:

• Ignoring empty stack check
• Changing return type incorrectly
• Making items public unnecessarily

5. Given a stack implemented using an array, how would you implement a function reverseStack that reverses the elements in the stack without using any extra stack or array? Which approach is correct?

hard

A. Use recursion to pop all elements, then insert them at bottom recursively

B. Use a for loop to swap elements from start and end indices

C. Pop all elements into a new array, then push back in reverse order

D. Use a queue to dequeue and enqueue elements to reverse order

Solution

1. Step 1: Understand constraints

   No extra stack or array allowed, so iterative swapping or extra storage is not possible.

2. Step 2: Use recursion to reverse stack

   Recursion can pop all elements, then insert each element at the bottom of the stack recursively, effectively reversing it.

3. Step 3: Evaluate other options

   Use a for loop to swap elements from start and end indices requires direct index access which is not typical for stack abstraction. Pop all elements into a new array, then push back in reverse order uses extra array. Use a queue to dequeue and enqueue elements to reverse order uses queue, which is extra data structure.

4. Final Answer:

   Use recursion to pop all elements, then insert them at bottom recursively -> Option A

5. Quick Check:

   Recursion reverses stack without extra storage [OK]

Hint: Recursion can reverse stack by inserting at bottom [OK]

Common Mistakes:

• Using extra arrays or stacks against constraints
• Trying to swap elements directly in stack
• Using other data structures like queue