Double Ended Queue Deque in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new empty deque.

DSA Rust

let mut deque = std::collections::VecDeque::[1]();

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

Anew

Bwith_capacity

Cdefault

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using with_capacity requires a size argument.

Using from needs an existing collection.

Using default is valid but less explicit here.

The new() function creates an empty deque.

2fill in blank

medium

Complete the code to add an element to the front of the deque.

DSA Rust

deque.[1](10);

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

Apush_back

Binsert

Cpush_front

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using push_back adds to the back, not front.

Using insert requires an index argument.

Using append is for concatenating collections.

push_front() adds an element to the front of the deque.

3fill in blank

hard

Fix the error in the code to remove an element from the back of the deque.

DSA Rust

let last = deque.[1]().unwrap();

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

Aremove

Bpop_back

Cpop_front

Dtake

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front removes from the front.

Using remove requires an index argument.

Using take is not a method on VecDeque.

pop_back() removes and returns the last element from the deque.

4fill in blank

hard

Fill both blanks to check if the deque is empty and print a message.

DSA Rust

if deque.[1]() [2] {
    println!("Deque is empty");
}

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

Ais_empty

Bis_full

C== true

D!= false

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_full() which does not exist for VecDeque.

Using != false is correct but less clear here.

Omitting the comparison is valid but this task expects it.

is_empty() returns true if the deque has no elements. Comparing with == true confirms this condition.

5fill in blank

hard

Fill all three blanks to create a deque from a vector, add an element at the back, and remove from the front.

DSA Rust

let mut deque = std::collections::VecDeque::[1](vec![1, 2, 3]);
deque.[2](4);
let first = deque.[3]().unwrap();

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

Afrom

Bpush_back

Cpop_front

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using new() instead of from() to create from vector.

Using push_front() instead of push_back().

Using pop_back() instead of pop_front().

from() creates a deque from a vector. push_back() adds an element at the end. pop_front() removes the first element.

Practice

1. What is a deque in data structures?

easy

A. A double-ended queue allowing insertion and removal from both ends

B. A queue that only allows insertion at the front

C. A stack that only allows removal from the top

D. A list that only supports random access

Solution

1. Step 1: Understand the deque concept

   A deque is a data structure where elements can be added or removed from both the front and the back.

2. Step 2: Compare options with deque definition

   Only double-ended queue allowing insertion and removal from both ends correctly describes this behavior; others describe different structures.

3. Final Answer:

   A double-ended queue allowing insertion and removal from both ends -> Option A

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing deque with stack or regular queue
• Thinking deque only allows insertion at one end
• Assuming deque is a fixed-size array

2. Which of the following is the correct way to create an empty VecDeque in Rust?

easy

A. let dq = VecDeque::empty();

B. let dq = VecDeque();

C. let dq = new VecDeque();

D. let dq = VecDeque::new();

Solution

1. Step 1: Recall Rust VecDeque creation syntax

   In Rust, the standard way to create an empty VecDeque is by calling VecDeque::new().

2. Step 2: Check each option for Rust syntax correctness

   let dq = VecDeque::new(); uses correct Rust syntax. The other options are invalid Rust syntax for VecDeque creation.

3. Final Answer:

   let dq = VecDeque::new(); -> Option D

4. Quick Check:

   VecDeque::new() creates empty deque [OK]

Hint: Use VecDeque::new() to create empty deque in Rust [OK]

Common Mistakes:

• Using constructor syntax like VecDeque() which is invalid in Rust
• Trying to call new as a method without ::
• Using non-existent empty() method

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

use std::collections::VecDeque;

fn main() {
    let mut dq = VecDeque::new();
    dq.push_back(10);
    dq.push_front(20);
    dq.push_back(30);
    dq.pop_front();
    println!("{:?}", dq);
}

medium

A. [10, 30]

B. [20, 10, 30]

C. [30, 10]

D. [10, 20, 30]

Solution

1. Step 1: Trace deque operations step-by-step

   Initially empty deque: [] - push_back(10): [10] - push_front(20): [20, 10] - push_back(30): [20, 10, 30] - pop_front(): removes 20, resulting in [10, 30]

2. Step 2: Confirm final deque state

   After all operations, the deque contains [10, 30].

3. Final Answer:

   [10, 30] -> Option A

4. Quick Check:

   After pop_front, deque = [10, 30] [OK]

Hint: Track push/pop order carefully front and back [OK]

Common Mistakes:

• Forgetting pop_front removes front element
• Mixing order of push_front and push_back
• Assuming pop_front removes back element

4. Identify the error in the following Rust code using VecDeque:

use std::collections::VecDeque;

fn main() {
    let mut dq = VecDeque::new();
    dq.push_front(5);
    dq.push_back(10);
    let x = dq.pop_back();
    println!("{}", x);
}

medium

A. pop_back() does not return a value

B. Cannot print Option<i32> directly with {}

C. VecDeque does not have push_front method

D. dq must be immutable to pop elements

Solution

1. Step 1: Understand pop_back return type

   pop_back() returns an Option containing the popped value or None if empty.

2. Step 2: Check println! usage

   Using {} tries to print the Option<i32> directly, which causes a type mismatch error. {:?} or unwrap() is needed.

3. Final Answer:

   Cannot print Option<i32> directly with {} -> Option B

4. Quick Check:

   Option<i32> needs {:?} or unwrap() to print [OK]

Hint: Use {:?} or unwrap() to print Option values [OK]

Common Mistakes:

• Trying to print Option with {} instead of {:?}
• Assuming pop_back returns value directly, not Option
• Thinking VecDeque lacks push_front method

5. You want to implement a sliding window maximum using a VecDeque in Rust. Which approach correctly maintains the deque to always have the maximum at the front while sliding the window?

hard

A. Remove elements from front smaller than new element before pushing it to front

B. Always push new element to front without removing any elements

C. Remove elements from back smaller than new element before pushing it to back

D. Remove elements from back larger than new element before pushing it to back

Solution

1. Step 1: Understand sliding window max logic with deque

   To keep max at front, remove all smaller elements from back before adding new element at back.

2. Step 2: Analyze options for correct deque maintenance

   Remove elements from back smaller than new element before pushing it to back correctly describes removing smaller elements from back before pushing new element to back. Others do not maintain max at front properly.

3. Final Answer:

   Remove elements from back smaller than new element before pushing it to back -> Option C

4. Quick Check:

   Sliding window max uses back removals of smaller elements [OK]

Hint: Pop smaller from back before push_back new max candidate [OK]

Common Mistakes:

• Removing from front instead of back
• Pushing new elements to front incorrectly
• Removing larger elements instead of smaller