Pop Operation on Stack in DSA Rust - 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. If the stack is empty, it usually returns an error or None.

beginner

In Rust, which method is commonly used to perform a pop operation on a Vec used as a stack?

The pop() method removes and returns the last element of the Vec, acting as the top of the stack.

beginner

What happens if you call pop() on an empty stack in Rust?

It returns None, indicating there is no element to remove.

beginner

Why is the pop operation considered Last In, First Out (LIFO)?

Because the last item added (pushed) to the stack is the first one to be removed (popped).

beginner

Show a simple Rust code snippet that pops an element from a stack and prints it.

let mut stack = vec![1, 2, 3];
if let Some(top) = stack.pop() {
    println!("Popped: {}", top);
} else {
    println!("Stack is empty");
}

What does the pop operation return when the stack is empty?

A The last element

B Zero

C An error or None

D The first element

Pop returns None or an error if the stack is empty because there is no element to remove.

Which data structure principle does pop follow?

A First In, First Out (FIFO)

B Random Access

C Priority Queue

D Last In, First Out (LIFO)

Pop removes the last added item first, following LIFO.

In Rust, which method removes the top element from a vector used as a stack?

A pop()

B remove()

C push()

D insert()

The pop() method removes and returns the last element of a vector.

What happens to the stack size after a pop operation?

A It stays the same

B It decreases by one

C It increases by one

D It doubles

Pop removes the top element, so the stack size decreases by one.

If a stack has elements [1, 2, 3], what will be the top element after one pop?

A 2

B 1

C 3

D None

Pop removes 3, so 2 becomes the new top.

Explain how the pop operation works on a stack and what happens when the stack is empty.

Think about removing the last added item and what happens if there is nothing to remove.

Write a simple Rust code snippet that demonstrates popping an element from a stack and handling the empty case.

Use if let Some(...) to handle the pop result.

Practice

1. What does the pop operation do in a stack data structure?

easy

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

B. Removes and returns the top element 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, which is the last inserted item.

3. Final Answer:

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

4. Quick Check:

   Pop = remove top element [OK]

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

Common Mistakes:

• Thinking pop removes from bottom
• Confusing pop with push
• Assuming pop returns without removing

2. Which of the following is the correct Rust syntax to pop an element from a stack implemented as a Vec<T> named stack?

easy

A. let x = stack.pop_front();

B. let x = stack.remove();

C. let x = stack.pop();

D. let x = stack.delete();

Solution

1. Step 1: Recall Rust Vec methods

   Rust's Vec has a pop() method that removes and returns the last element.

2. Step 2: Check other options

   remove() requires an index, pop_front() is for other collections, delete() does not exist.

3. Final Answer:

   let x = stack.pop(); -> Option C

4. Quick Check:

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

Hint: Use stack.pop() to remove last element in Rust Vec [OK]

Common Mistakes:

• Using pop_front() which is not in Vec
• Calling remove() without index
• Using non-existent delete() method

3. What is the output of the following Rust code?

let mut stack = vec![10, 20, 30];
let popped = stack.pop();
println!("{:?}", popped);
println!("{:?}", stack);

medium

A. Some(30)
[10, 20]

B. 30
[10, 20, 30]

C. None
[10, 20, 30]

D. Some(20)
[10, 30]

Solution

1. Step 1: Understand pop() return type

   pop() returns an Option containing the last element if present, here Some(30).

2. Step 2: Check stack after pop

   After popping 30, stack becomes [10, 20].

3. Final Answer:

   Some(30) [10, 20] -> Option A

4. Quick Check:

   pop returns Some(last element), stack shrinks [OK]

Hint: pop() returns Some(value) if not empty [OK]

Common Mistakes:

• Expecting pop() to return value directly, not Option
• Not updating stack after pop
• Confusing popped element with stack contents

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

let mut stack: Vec<i32> = Vec::new();
let x = stack.pop().unwrap();
println!("{}", x);

medium

A. pop() method does not exist on Vec

B. println! macro syntax is incorrect

C. stack is not mutable

D. Calling unwrap() on None causes panic

Solution

1. Step 1: Check pop() on empty Vec

   pop() returns None if the Vec is empty.

2. Step 2: Understand unwrap() behavior

   Calling unwrap() on None causes a runtime panic (crash).

3. Final Answer:

   Calling unwrap() on None causes panic -> Option D

4. Quick Check:

   unwrap on None = panic [OK]

Hint: Avoid unwrap() on pop() without checking Some [OK]

Common Mistakes:

• Assuming pop() always returns Some
• Ignoring mut keyword needed for pop
• Misreading println! syntax

5. Given a stack implemented as Vec<i32>, how can you safely pop all elements and collect them into a new vector in reverse order without panicking?

hard

A. Use a loop with while let Some(x) = stack.pop() and push x into new vector

B. Use stack.pop().unwrap() repeatedly until empty

C. Use stack.remove(0) repeatedly to pop elements

D. Use stack.pop_front() to remove elements safely

Solution

1. Step 1: Avoid unwrap() to prevent panic

   Using unwrap() on pop() can panic if stack is empty, so avoid it.

2. Step 2: Use while let to safely pop

   Using while let Some(x) = stack.pop() safely extracts elements until empty.

3. Step 3: Push popped elements into new vector

   Each popped element x is pushed into a new vector, reversing order.

4. Final Answer:

   Use a loop with while let Some(x) = stack.pop() and push x into new vector -> Option A

5. Quick Check:

   Safe pop loop = while let Some(x) = pop() [OK]

Hint: Use while let Some(x) = pop() to avoid panic [OK]

Common Mistakes:

• Using unwrap() causing panic on empty stack
• Using remove(0) which is inefficient
• Calling pop_front() which does not exist on Vec