Why Stack Exists and What Problems It Solves in DSA Typescript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA Typescript

stack.[1](5);

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

Apush

Bpop

Cpeek

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which removes an element instead of adding.

Using 'peek' which only views the top element.

The push method adds an element to the top of the stack.

2fill in blank

medium

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

DSA Typescript

const top = stack.[1]();

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

Aunshift

Bpush

Cpeek

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' which adds instead of removes.

Using 'peek' which only views without removing.

The pop method removes and returns the top element of the stack.

3fill in blank

hard

Fix the error in the code to check the top element without removing it.

DSA Typescript

const top = stack.[1]();

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

Apeek

Bpush

Cpop

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which removes the element.

Using 'push' which adds an element.

The peek method returns the top element without removing it.

4fill in blank

hard

Fill both blanks to create a stack and add an element.

DSA Typescript

const stack = new [1]();
stack.[2](10);

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

AStack

BQueue

Cpush

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Queue' which is a different data structure.

Using 'pop' to add an element instead of 'push'.

We create a new Stack and use push to add an element.

5fill in blank

hard

Fill the two blanks to remove the top element and check the new top.

DSA Typescript

stack.[1]();
const newTop = stack.[2]();
console.log(newTop);

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

Apush

Bpop

Cpeek

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'push' to remove an element.

Using 'pop' twice without peeking.

First, pop removes the top element. Then, peek checks the new top element. Finally, we print it.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To sort numbers in ascending order automatically

B. To store data in random order for quick access

C. To keep track of function calls and return in the correct order

D. To store large amounts of data permanently

Solution

1. Step 1: Understand the purpose of a stack

   A stack stores items in Last In First Out (LIFO) order, which is perfect for tracking function calls.

2. Step 2: Relate stack to function calls

   When a function calls another, the current function is paused and saved on the stack. When the called function finishes, the program returns to the last saved function.

3. Final Answer:

   To keep track of function calls and return in the correct order -> Option C

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack follows LIFO, perfect for tracking nested calls [OK]

Common Mistakes:

• Thinking stack stores data randomly
• Confusing stack with queue
• Assuming stack sorts data automatically

2. Which of the following is the correct way to push an element '5' onto a stack named myStack in TypeScript?

easy

A. myStack.push(5);

B. myStack.add(5);

C. myStack.insert(5);

D. myStack.enqueue(5);

Solution

1. Step 1: Recall stack method names in TypeScript

   Stacks use push() to add elements on top.

2. Step 2: Identify correct method for adding element

   push() is the standard method; others like add(), insert(), or enqueue() are not used for stacks.

3. Final Answer:

   myStack.push(5); -> Option A

4. Quick Check:

   Push adds element on top [OK]

Hint: Use push() to add on stack top [OK]

Common Mistakes:

• Using enqueue which is for queues
• Using add or insert which are not stack methods
• 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();
stack.push(30);
console.log(stack);

medium

A. [30]

B. [10, 20, 30]

C. [20, 30]

D. [10, 30]

Solution

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

   Initially stack is empty []. Push 10 -> [10]. Push 20 -> [10, 20]. Pop removes 20 -> [10]. Push 30 -> [10, 30].

2. Step 2: Determine final stack content

   After all operations, stack contains [10, 30].

3. Final Answer:

   [10, 30] -> Option D

4. Quick Check:

   Push/pop changes stack top [OK]

Hint: Pop removes last pushed element [OK]

Common Mistakes:

• Forgetting pop removes top element
• Assuming pop returns stack without removing
• Mixing order of push and pop

4. Find the error in this TypeScript stack usage code:

const stack: number[] = [];
stack.push(1);
stack.push(2);
const top = stack.peek();
console.log(top);

medium

A. Stack does not have a peek() method by default

B. push() method is used incorrectly

C. stack should be declared as Stack<number> not number[]

D. console.log cannot print stack elements

Solution

1. Step 1: Check methods available on array used as stack

   TypeScript arrays have push() and pop(), but no peek() method by default.

2. Step 2: Identify cause of error

   Calling stack.peek() will cause a runtime error because peek() is not defined.

3. Final Answer:

   Stack does not have a peek() method by default -> Option A

4. Quick Check:

   peek() missing in array stack [OK]

Hint: Array stack has pop(), no peek() [OK]

Common Mistakes:

• Assuming peek() exists on arrays
• Thinking push() is wrong
• Confusing type declaration with method error

5. You want to reverse a string using a stack in TypeScript. Which approach correctly uses a stack to reverse the string "hello"?

hard

A. Push characters and pop in same order to keep string unchanged

B. Push each character onto stack, then pop all to build reversed string

C. Use stack to sort characters alphabetically

D. Push entire string onto stack, then pop once to get reversed string

Solution

1. Step 1: Understand stack reversal logic

   Pushing characters one by one stores them in order. Popping them reverses the order because stack is LIFO.

2. Step 2: Apply to string reversal

   Push 'h', 'e', 'l', 'l', 'o' then pop all to get 'o', 'l', 'l', 'e', 'h' which is reversed string.

3. Final Answer:

   Push each character onto stack, then pop all to build reversed string -> Option B

4. Quick Check:

   Stack LIFO reverses order [OK]

Hint: Push chars, pop all to reverse string [OK]

Common Mistakes:

• Pushing entire string at once
• Popping in same order as push
• Using stack for sorting instead of reversing