Implement Stack Using Queue in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element into the stack using a queue.

DSA Rust

fn push(&mut self, x: i32) {
    self.queue.push_back(x);
    for _ in 0..self.queue.len() - 1 {
        let val = self.queue.pop_front().unwrap();
        self.queue.[1](val);
    }
}

Drag options to blanks, or click blank then click option'

Apush_front

Bpop_back

Cpush_back

Dpop_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_back instead of push_back causes errors.

Using push_front changes the order incorrectly.

We use push_back to add the element at the back of the queue after popping from the front to rotate the queue.

2fill in blank

medium

Complete the code to pop the top element from the stack.

DSA Rust

fn pop(&mut self) -> Option<i32> {
    self.queue.[1]()
}

Drag options to blanks, or click blank then click option'

Apop

Bpop_back

Cremove

Dpop_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_back returns the wrong element.

Using remove requires an index and is not suitable here.

We pop from the front of the queue because the push operation rotates the queue to keep the top element at the front.

3fill in blank

hard

Fix the error in the peek method to return the top element without removing it.

DSA Rust

fn top(&self) -> Option<i32> {
    self.queue.[1]().copied()
}

Drag options to blanks, or click blank then click option'

Aget(0)

Bfirst

Cfront

Dpeek_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front removes the element.

Using get(0) returns an Option<&T>, but first() is more idiomatic.

The first() method returns an Option reference to the first element without removing it.

4fill in blank

hard

Fill both blanks to check if the stack is empty.

DSA Rust

fn empty(&self) -> bool {
    self.queue.[1]() [2] 0
}

Drag options to blanks, or click blank then click option'

Alen

B==

C!=

Dis_empty

Attempts:

3 left

💡 Hint

Common Mistakes

Using len() != 0 returns the opposite of empty.

Using is_empty() == 0 is incorrect because is_empty() returns a bool.

We use len() == 0 to check if the queue has no elements. Using is_empty() directly is better, but the code syntax requires comparison here.

5fill in blank

hard

Fill all three blanks to implement the push method that rotates the queue correctly.

DSA Rust

fn push(&mut self, x: i32) {
    self.queue.[1](x);
    for _ in 0..self.queue.[2]() - 1 {
        let val = self.queue.[3]().unwrap();
        self.queue.push_back(val);
    }
}

Drag options to blanks, or click blank then click option'

Apush_back

Blen

Cpop_front

Dpush_front

Attempts:

3 left

💡 Hint

Common Mistakes

Using push_front instead of push_back changes order incorrectly.

Using pop_back instead of pop_front breaks rotation.

We push_back the new element, then rotate by popping from the front and pushing back to keep the stack order.

Practice

1. What is the main idea behind implementing a stack using a queue?

easy

A. Use one or two queues to simulate stack operations like push and pop

B. Use a queue to sort elements in ascending order

C. Use a stack to implement queue operations

D. Use two stacks to implement a queue

Solution

1. Step 1: Understand stack and queue operations

   A stack follows Last-In-First-Out (LIFO), while a queue follows First-In-First-Out (FIFO).

2. Step 2: Simulate stack using queue(s)

   By using one or two queues and carefully managing element order, we can mimic stack push and pop operations.

3. Final Answer:

   Use one or two queues to simulate stack operations like push and pop -> Option A

4. Quick Check:

   Stack via queue = simulate push/pop with queues [OK]

Hint: Remember stack is LIFO, queue is FIFO; simulate LIFO with queue(s) [OK]

Common Mistakes:

• Confusing stack and queue order
• Trying to sort elements instead of simulating stack
• Using stack to implement queue instead

2. Which Rust code snippet correctly declares a queue using VecDeque for implementing a stack?

easy

A. let mut queue = Vec::with_capacity(10);

B. let queue = Vec<i32>::new();

C. let mut queue: VecDeque<i32> = VecDeque::new();

D. let queue: VecDeque = Vec::new();

Solution

1. Step 1: Identify correct queue type in Rust

   VecDeque is the standard double-ended queue in Rust, suitable for queue operations.

2. Step 2: Check correct syntax for mutable queue declaration

   "let mut queue: VecDeque<i32> = VecDeque::new();" correctly declares a mutable queue of i32.

3. Final Answer:

   let mut queue: VecDeque<i32> = VecDeque::new(); -> Option C

4. Quick Check:

   VecDeque mutable queue declaration = let mut queue: VecDeque<i32> = VecDeque::new(); [OK]

Hint: Use VecDeque with mut keyword for queue in Rust [OK]

Common Mistakes:

• Using Vec instead of VecDeque for queue
• Missing mut keyword for mutable queue
• Wrong type annotations

3. Given this Rust code implementing push for stack using one queue, what is the queue state after pushing 1, then 2?

use std::collections::VecDeque;

struct Stack {
    queue: VecDeque<i32>,
}

impl Stack {
    fn new() -> Self {
        Stack { queue: VecDeque::new() }
    }

    fn push(&mut self, x: i32) {
        self.queue.push_back(x);
        let size = self.queue.len();
        for _ in 0..size - 1 {
            if let Some(val) = self.queue.pop_front() {
                self.queue.push_back(val);
            }
        }
    }
}

fn main() {
    let mut stack = Stack::new();
    stack.push(1);
    stack.push(2);
    println!("{:?}", stack.queue);
}

medium

A. [2, 1]

B. [1, 2]

C. [1]

D. [2]

Solution

1. Step 1: Trace push(1) operation

   Push 1: queue = [1]. No rotation since size=1.

2. Step 2: Trace push(2) operation

   Push 2: queue = [1, 2]. Rotate size-1=1 times: pop_front 1, push_back 1 -> queue = [2, 1].

3. Final Answer:

   [2, 1] -> Option A

4. Quick Check:

   After push 1,2 queue = [2,1] [OK]

Hint: After push, rotate queue to front to simulate stack top [OK]

Common Mistakes:

• Not rotating queue after push
• Confusing push_back and pop_front order
• Assuming queue keeps insertion order

4. Identify the bug in this pop method for stack implemented using one queue:

fn pop(&mut self) -> Option<i32> {
    self.queue.pop_back()
}

medium

A. pop_back() is invalid for VecDeque

B. pop_back() removes from the wrong end; should use pop_front()

C. pop_back() is correct; no bug

D. pop_back() requires mutable borrow error

Solution

1. Step 1: Understand queue ends in VecDeque

   VecDeque supports pop_front() and pop_back(), but our stack top is at front after push rotation.

2. Step 2: Check which end holds stack top

   After push rotation, stack top is at front, so pop_front() removes top, not pop_back().

3. Final Answer:

   pop_back() removes from the wrong end; should use pop_front() -> Option B

4. Quick Check:

   Stack top at front, pop_front() needed [OK]

Hint: Pop from front after rotation to get stack top [OK]

Common Mistakes:

• Using pop_back() instead of pop_front()
• Not rotating queue after push
• Confusing queue ends

5. How would you implement the top method for a stack using a single queue in Rust, assuming push rotates the queue so the top is at the front?

hard

A. Return the last element using queue.back().cloned()

B. Remove and return the last element using queue.pop_back()

C. Remove and return the first element using queue.pop_front()

D. Return the element at the front of the queue without removing it using queue.front().cloned()

Solution

1. Step 1: Recall queue rotation after push

   After push, queue is rotated so top element is at front.

2. Step 2: Access top without removing

   Use queue.front().cloned() to get top element without popping.

3. Final Answer:

   Return the element at the front of the queue without removing it using queue.front().cloned() -> Option D

4. Quick Check:

   Top element at front, use front().cloned() [OK]

Hint: Top is front element after rotation; use front().cloned() [OK]

Common Mistakes:

• Using pop_front() which removes element
• Using back() which is bottom of stack
• Removing element instead of just viewing