Pop Operation on Stack in DSA Javascript - Time & Space Complexity

Time Complexity: Pop Operation on Stack

O(1)

Understanding Time Complexity

Let's explore how long it takes to remove an item from a stack using the pop operation.

We want to know how the time needed changes as the stack grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Stack {
  constructor() {
    this.items = [];
  }

  pop() {
    if (this.items.length === 0) return null;
    return this.items.pop();
  }
}
    

This code defines a stack and removes the top item when pop is called.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Removing the last element from the array.
• How many times: Once per pop call, no loops or recursion inside pop.

How Execution Grows With Input

Removing the top item takes the same effort no matter how big the stack is.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The time stays constant as the stack grows.

Final Time Complexity

Time Complexity: O(1)

This means popping an item takes the same short time no matter how many items are in the stack.

Common Mistake

[X] Wrong: "Pop takes longer if the stack is bigger because it has more items to check."

[OK] Correct: Pop only removes the last item directly without checking others, so size does not affect time.

Interview Connect

Understanding that pop is a quick, constant-time operation helps you explain stack efficiency clearly in interviews.

Self-Check

"What if the stack was implemented using a linked list instead of an array? How would the time complexity of pop change?"

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