Pop Operation on Stack in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the pop operation do in a stack?

It removes the top item from the stack and returns it, reducing the stack size by one.

beginner

What happens if you try to pop from an empty stack?

Usually, it causes an error or returns a special value (like null or undefined) because there is no item to remove.

beginner

In TypeScript, which data structure is commonly used to implement a stack for pop operation?

An array is commonly used, where pop() removes the last element, acting as the top of the stack.

beginner

Why is the pop operation considered LIFO?

Because it removes the last item added to the stack, following Last In, First Out order.

beginner

What is the time complexity of the pop operation on a stack?

The pop operation runs in constant time, O(1), because it only removes the top element.

What does the pop operation return when called on a non-empty stack?

A The top element of the stack

B The bottom element of the stack

C The size of the stack

D Nothing, it just removes the element

Pop removes and returns the top element of the stack.

What is the result of popping all elements from a stack one by one?

A Stack reverses order

B Stack size doubles

C Stack remains unchanged

D Stack becomes empty

Popping all elements removes them, leaving the stack empty.

Which data structure principle does pop operation follow?

A FIFO (First In First Out)

B Random Access

C LIFO (Last In First Out)

D Priority Queue

Pop follows LIFO, removing the last added element first.

What happens if you pop from an empty stack in most implementations?

A Throws an error or returns null/undefined

B Returns the first element

C Adds a new element

D Doubles the stack size

Popping from empty stack usually causes error or returns null/undefined.

What is the time complexity of the pop operation on a stack?

A O(n)

B O(1)

C O(n^2)

D O(log n)

Pop operation runs in constant time O(1).

Explain how the pop operation works on a stack and what happens to the stack after popping.

Think about removing the last added item and how the stack changes.

Describe what should happen if you try to pop from an empty stack and why handling this case is important.

Consider what happens when there is nothing to pop.

Practice

1. What does the pop() operation do in a stack?

easy

A. Clears all elements from the stack

B. Adds a new element to the bottom of the stack

C. Returns the bottom element without removing it

D. Removes and returns the top element of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is on top.

2. Step 2: Define pop operation

   The pop operation removes and returns this top element from the stack.

3. Final Answer:

   Removes and returns the top element of the stack -> Option D

4. Quick Check:

   pop() removes top element = A [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

2. Which of the following is the correct TypeScript syntax to pop an element from a stack array named stack?

easy

A. pop(stack);

B. stack.remove();

C. stack.pop();

D. stack.delete();

Solution

1. Step 1: Recall array pop method in TypeScript

   In TypeScript, arrays have a built-in method pop() to remove the last element.

2. Step 2: Match correct syntax

   The correct syntax to pop from stack is stack.pop();. Other options are invalid methods.

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Array pop method = stack.pop() [OK]

Hint: Use .pop() method on array to remove top stack element [OK]

Common Mistakes:

• Using non-existent methods like remove() or delete()
• Calling pop as a function with stack as argument
• Confusing pop with other array methods

3. What will be the output after executing the following TypeScript code?

const stack = [10, 20, 30];
const popped = stack.pop();
console.log(popped);
console.log(stack);

medium

A. 30
[10, 20]

B. 20
[10, 20, 30]

C. 30
[10, 20, 30]

D. undefined
[10, 20]

Solution

1. Step 1: Understand pop() effect on array

   pop() removes and returns the last element. Here, 30 is removed and returned.

2. Step 2: Check console outputs

   First console.log prints popped value 30. Second prints updated stack [10, 20].

3. Final Answer:

   30 [10, 20] -> Option A

4. Quick Check:

   pop() returns last element and removes it [OK]

Hint: pop() returns last item and removes it from array [OK]

Common Mistakes:

• Expecting pop() to return second last element
• Thinking pop() does not modify the array
• Confusing pop() return value with stack after pop

4. Identify the error in the following TypeScript code that tries to pop an element from an empty stack:

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

medium

A. pop() returns undefined when stack is empty, no error

B. Runtime error because pop() on empty array is invalid

C. Syntax error due to missing type annotation

D. pop() removes first element instead of last

Solution

1. Step 1: Check pop() behavior on empty array

   pop() returns undefined if the array is empty; it does not throw an error.

2. Step 2: Verify code correctness

   Code runs without error and prints undefined.

3. Final Answer:

   pop() returns undefined when stack is empty, no error -> Option A

4. Quick Check:

   pop() on empty array = undefined [OK]

Hint: pop() returns undefined if stack is empty, no crash [OK]

Common Mistakes:

• Expecting runtime error on empty pop()
• Confusing pop() with shift() behavior
• Thinking pop() removes first element

5. Given a stack implemented as an array stack = [5, 10, 15, 20], write a TypeScript function popTwo that pops two elements and returns them as an array. What will be the returned value and the stack after calling popTwo(stack)?

hard

A. [15, 20], stack = [5, 10]

B. [20, 15], stack = [5, 10]

C. [20, 15], stack = [10, 5]

D. [10, 5], stack = [15, 20]

Solution

1. Step 1: Understand pop() order and stack state

   pop() removes last element first. First pop returns 20, second pop returns 15.

2. Step 2: Determine returned array and stack after two pops

   Returned array is [20, 15]. Stack after pops is [5, 10].

3. Final Answer:

   [20, 15], stack = [5, 10] -> Option B

4. Quick Check:

   Two pops remove last two elements in order [20, 15] [OK]

Hint: Pop twice returns last two elements in reverse order [OK]

Common Mistakes:

• Returning popped elements in wrong order
• Assuming stack order changes after pop
• Mixing up stack contents after pops