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

Recall & Review

beginner

What does the pop operation do in a stack?

The pop operation removes the top item from the stack and returns it. It follows the Last In, First Out (LIFO) rule.

beginner

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

Trying to pop from an empty stack usually results in an error or returns a special value like null or undefined, indicating the stack is empty.

beginner

In JavaScript, which array method is commonly used to implement the pop operation on a stack?

The pop() method removes the last element from an array and returns it, making it perfect for stack pop operations.

intermediate

Explain why the pop operation is considered constant time (O(1)) in a stack.

Pop removes the top element directly without needing to look through other elements, so it takes the same amount of time regardless of stack size.

beginner

What is the state of the stack after popping the top element from 1 -> 2 -> 3 -> null?

After popping, the stack becomes 1 -> 2 -> null. The top element (3) is removed.

What does the pop operation remove from a stack?

A The top element

B The bottom element

C A random element

D All elements

Pop always removes the top element of the stack.

What will happen if you pop from an empty stack?

A Return null or undefined

B Nothing happens

C Add a new element

D Return the bottom element

Popping from an empty stack returns null or undefined or throws an error.

Which JavaScript array method is used to pop an element?

A push()

B pop()

C shift()

D unshift()

The pop() method removes the last element from an array.

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

A O(n)

B O(log n)

C O(1)

D O(n^2)

Pop is O(1) because it removes the top element directly.

If the stack is 5 -> 10 -> 15 -> null, what is the stack after one pop?

A Empty stack

B 10 -> 15 -> null

C 5 -> 15 -> null

D 5 -> 10 -> null

Pop removes the top element (15), leaving 5 -> 10 -> null.

Describe the pop operation on a stack and what happens to the stack after popping.

Think about the last item added and what happens when you remove it.

Explain why pop operation on a stack is efficient and what its time complexity is.

Consider how many steps it takes to remove the top element regardless of stack size.

Practice

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

easy

A. Removes and returns the top element of the stack

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

C. Returns the bottom element without removing it

D. Clears all elements from 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 the top element of the stack, following LIFO.

3. Final Answer:

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

4. Quick Check:

   pop() = remove top element [OK]

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

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek (which only returns)
• Assuming pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack implemented as an array in JavaScript?

easy

A. pop(stack);

B. stack.popElement();

C. stack.pop();

D. stack.remove();

Solution

1. Step 1: Recall JavaScript array pop method

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

2. Step 2: Match correct syntax

   The correct syntax to pop from an array named stack is stack.pop();

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Use array.pop() to remove last element [OK]

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

Common Mistakes:

• Using non-existent methods like popElement()
• Calling pop as a standalone function
• Using remove() which is not valid for arrays

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

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

medium

A. 20
[10, 20, 30]

B. undefined
[10, 20]

C. 30
[10, 20, 30]

D. 30
[10, 20]

Solution

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

   pop() removes the last element (30) and returns it, so popped is 30.

2. Step 2: Check stack after pop

   After pop, stack becomes [10, 20] because 30 is removed.

3. Final Answer:

   30 [10, 20] -> Option D

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
• Assuming stack remains unchanged after pop
• Confusing pop() return value with stack content

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

let stack = [1, 2, 3];
let popped = stack.pop;
console.log(popped);

medium

A. pop is not a function

B. pop method is not called, missing parentheses

C. stack is not defined

D. pop returns undefined always

Solution

1. Step 1: Check how pop is used

   The code assigns stack.pop without parentheses, so it assigns the function itself, not the result.

2. Step 2: Correct usage of pop

   To pop and get the value, pop must be called with parentheses: stack.pop().

3. Final Answer:

   pop method is not called, missing parentheses -> Option B

4. Quick Check:

   Use stack.pop() to call pop method [OK]

Hint: Always use parentheses to call pop() method [OK]

Common Mistakes:

• Forgetting parentheses when calling pop
• Thinking pop is a property, not a function
• Assuming pop returns undefined without call

5. Given a stack implemented as an array stack = [5, 10, 15], what will be the state of the stack after performing two consecutive pop() operations?

hard

A. [5]

B. [10]

C. [5, 10]

D. []

Solution

1. Step 1: Perform first pop()

   First pop removes 15, stack becomes [5, 10].

2. Step 2: Perform second pop()

   Second pop removes 10, stack becomes [5].

3. Final Answer:

   [5] -> Option A

4. Quick Check:

   Two pops remove last two elements [OK]

Hint: Each pop removes one top element [OK]

Common Mistakes:

• Removing elements from the bottom instead of top
• Assuming pop does not change stack
• Confusing number of pops with elements removed