Push Operation on Stack in DSA Rust - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on the stack?

Consider the following Rust code that pushes elements onto a stack implemented using a vector. What will be printed after all push operations?

DSA Rust

fn main() {
    let mut stack = Vec::new();
    stack.push(10);
    stack.push(20);
    stack.push(30);
    println!("{:?}", stack);
}

A []

B [10, 20, 30]

C [30, 20, 10]

D [10, 30, 20]

💡 Hint

Remember that push adds elements to the end of the vector.

The push method adds elements to the end of the vector. So the stack vector will contain elements in the order they were pushed: 10, then 20, then 30.

❓ Predict Output

intermediate

What is the stack state after pushing and popping elements?

Given the Rust code below, what is the final state of the stack vector after all operations?

DSA Rust

fn main() {
    let mut stack = Vec::new();
    stack.push(5);
    stack.push(15);
    stack.pop();
    stack.push(25);
    println!("{:?}", stack);
}

A [5, 25]

B [15, 25]

C [5, 15, 25]

D [25]

💡 Hint

Pop removes the last element pushed.

The pop removes the last pushed element (15). So after popping, the stack has [5]. Then 25 is pushed, resulting in [5, 25].

🔧 Debug

advanced

Why does this Rust stack push code fail to compile?

Identify the error in the following Rust code that tries to push elements onto a stack and fix it.

DSA Rust

fn main() {
    let stack = Vec::new();
    stack.push(1);
    println!("{:?}", stack);
}

A Error: missing semicolon after push

B No error, code runs fine

C Error: println! macro syntax incorrect

D Error: cannot borrow immutable variable `stack` as mutable

💡 Hint

Check if the vector is mutable before pushing.

The vector stack is declared immutable by default. You cannot push to an immutable vector. Declaring it as let mut stack = Vec::new(); fixes the error.

🧠 Conceptual

advanced

What is the time complexity of the push operation on a Rust Vec used as a stack?

When using a Rust vector as a stack, what is the average time complexity of the push operation?

A O(log n) time

B O(n) time for every push

C O(1) amortized time

D O(n^2) time

💡 Hint

Consider how vectors resize internally.

Push on a vector is usually O(1) amortized because resizing happens occasionally, but most pushes are constant time.

❓ Predict Output

expert

What is the output after pushing elements in a loop on a Rust stack?

Analyze the Rust code below. What will be printed after the loop completes?

DSA Rust

fn main() {
    let mut stack = Vec::new();
    for i in 1..=4 {
        stack.push(i * 2);
    }
    println!("{:?}", stack);
}

A [2, 4, 6, 8]

B [1, 2, 3, 4]

C [8, 6, 4, 2]

D [4, 8, 12, 16]

💡 Hint

Check the multiplication inside the loop.

The loop pushes i * 2 for i from 1 to 4, so the stack contains [2, 4, 6, 8].

Practice

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

easy

A. Removes the top element from the stack

B. Adds an element to the top of the stack

C. Returns the bottom element of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, where elements are added or removed from the top.

2. Step 2: Define push operation

   The push operation adds a new element on top of the existing elements in the stack.

3. Final Answer:

   Adds an element to the top of the stack -> Option B

4. Quick Check:

   Push = Add top element [OK]

Hint: Push always adds to the top of the stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Assuming push adds to the bottom

2. Which of the following is the correct Rust syntax to push an element 5 onto a stack implemented as a Vec<i32> named stack?

easy

A. stack.push(5);

B. stack.add(5);

C. stack.insert(5);

D. stack.append(5);

Solution

1. Step 1: Recall Rust Vec push method

   In Rust, Vec has a method push to add elements at the end (top of stack).

2. Step 2: Check method names

   add, insert, and append have different meanings or require different parameters.

3. Final Answer:

   stack.push(5); -> Option A

4. Quick Check:

   Vec push method = stack.push(5); [OK]

Hint: Use .push() to add elements to Vec stack [OK]

Common Mistakes:

• Using .add() which does not exist for Vec
• Confusing .insert() which needs index
• Using .append() which merges vectors

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

let mut stack = Vec::new();
stack.push(10);
stack.push(20);
stack.push(30);
println!("{:?}", stack);

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 20]

D. Compilation error

Solution

1. Step 1: Trace push operations

   Elements 10, 20, and 30 are pushed in order, so stack contains [10, 20, 30].

2. Step 2: Understand Vec print format

   Printing with {:?} shows elements in insertion order, so output is [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   Push order preserved in Vec print [OK]

Hint: Vec prints elements in insertion order [OK]

Common Mistakes:

• Assuming stack prints top element first
• Confusing push order with pop order
• Expecting reversed output

4. Identify the error in this Rust code that tries to push an element onto a stack:

let stack = Vec::new();
stack.push(100);

medium

A. Missing semicolon after push

B. Vec::new() is incorrect syntax

C. push method does not exist for Vec

D. stack is immutable, cannot push

Solution

1. Step 1: Check variable mutability

   The variable stack is declared immutable by default, so push cannot modify it.

2. Step 2: Understand push method requirements

   push requires the vector to be mutable to add elements.

3. Final Answer:

   stack is immutable, cannot push -> Option D

4. Quick Check:

   Mutable vector needed for push [OK]

Hint: Declare stack as mutable with 'mut' to push [OK]

Common Mistakes:

• Forgetting to add 'mut' keyword
• Thinking Vec::new() is wrong syntax
• Assuming push is not a Vec method

5. You want to implement a stack using a Rust Vec<i32> and push elements 1, 2, 3 in that order. After pushing, you want to print the stack from top to bottom. Which code snippet correctly achieves this?

hard

A. let mut stack = Vec::new();
stack.push(1);
stack.push(2);
stack.push(3);
for i in stack.iter() {
println!("{}", i);
}

B. let mut stack = Vec::new();
stack.push(3);
stack.push(2);
stack.push(1);
for i in stack.iter() {
println!("{}", i);
}

C. let mut stack = Vec::new();
stack.push(1);
stack.push(2);
stack.push(3);
for i in stack.iter().rev() {
println!("{}", i);
}

D. let mut stack = Vec::new();
stack.push(1);
stack.push(2);
stack.push(3);
for i in stack.into_iter() {
println!("{}", i);
}

Solution

1. Step 1: Push elements in order

   Elements 1, 2, 3 are pushed in order, so stack is [1, 2, 3] with 3 at top.

2. Step 2: Print from top to bottom

   To print from top (last pushed) to bottom, iterate in reverse order using iter().rev().

3. Final Answer:

   Code snippet with stack.iter().rev() -> Option C

4. Quick Check:

   Reverse iteration prints top to bottom [OK]

Hint: Use .iter().rev() to print stack top to bottom [OK]

Common Mistakes:

• Printing in normal order shows bottom to top
• Pushing elements in reverse order
• Using into_iter() which consumes the stack