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

Time Complexity: Pop Operation on Stack

O(1)

Understanding Time Complexity

We want to understand how long it takes to remove an item from a stack.

Specifically, how the time changes as the stack grows bigger.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Stack {
  private items: number[] = [];

  pop(): number | undefined {
    if (this.items.length === 0) return undefined;
    return this.items.pop();
  }
}

This code removes the top item from the stack if it exists.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Removing the last item from the internal array.
• How many times: This happens once per pop call, no loops or recursion involved.

How Execution Grows With Input

Removing the top item takes the same amount of time no matter how big the stack is.

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

Pattern observation: The time stays constant regardless of stack size.

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 to move many items."

[OK] Correct: The pop operation only removes the last item without shifting others, so it always takes the same short time.

Interview Connect

Knowing 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. 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