Doubly Linked List Structure and Node Design in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a doubly linked list node struct with data and pointers.

DSA Rust

struct Node<T> {
    data: T,
    prev: Option<Box<Node<T>>>,
    [1]: Option<Box<Node<T>>>,
}

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

Alink

Bnext

Cpointer

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect field names like 'value' or 'pointer' instead of 'next'.

Confusing 'prev' and 'next' pointers.

The next field points to the next node in the doubly linked list.

2fill in blank

medium

Complete the code to initialize a new node with given data and no links.

DSA Rust

impl<T> Node<T> {
    fn new(data: T) -> Self {
        Node {
            data,
            prev: None,
            [1]: None,
        }
    }
}

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

Avalue

Bpointer

Cnext

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the wrong field to None or forgetting to initialize next.

Using incorrect field names.

The next pointer is set to None when creating a new node with no next node.

3fill in blank

hard

Fix the error in the method that sets the next node pointer.

DSA Rust

impl<T> Node<T> {
    fn set_next(&mut self, next_node: Box<Node<T>>) {
        self.[1] = Some(next_node);
    }
}

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

Anext

Bprev

Cdata

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning to prev instead of next.

Trying to assign directly to data.

The method sets the next pointer to the given node wrapped in Some.

4fill in blank

hard

Fill both blanks to implement a method that returns true if the node has a next node.

DSA Rust

impl<T> Node<T> {
    fn has_next(&self) -> bool {
        self.[1].[2]()
    }
}

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

Anext

Bis_some

Cis_none

Dunwrap

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_none() which returns true if there is no next node.

Checking the wrong field like prev.

The method checks if the next pointer is Some, meaning there is a next node.

5fill in blank

hard

Fill all three blanks to implement a method that returns the data of the next node if it exists.

DSA Rust

impl<T: Clone> Node<T> {
    fn next_data(&self) -> Option<T> {
        self.[1].as_ref().map(|node| node.[2].[3]())
    }
}

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

Anext

Bdata

Cprev

Dclone

Attempts:

3 left

💡 Hint

Common Mistakes

Accessing prev instead of next.

Forgetting to clone the data, causing ownership errors.

The method accesses the next node, then clones its data to return it safely.

Practice

1. What is the main advantage of a doubly linked list over a singly linked list?

easy

A. It allows traversal in both forward and backward directions

B. It uses less memory per node

C. It requires fewer pointers per node

D. It can only be traversed forward

Solution

1. Step 1: Understand node pointers in doubly linked list

   A doubly linked list node has two pointers: one to the next node and one to the previous node.

2. Step 2: Compare traversal capabilities

   This allows moving forward and backward through the list, unlike singly linked lists which only allow forward traversal.

3. Final Answer:

   It allows traversal in both forward and backward directions -> Option A

4. Quick Check:

   Doubly linked list = two-way traversal [OK]

Hint: Remember: two pointers mean two directions [OK]

Common Mistakes:

• Thinking it uses less memory than singly linked list
• Assuming it only allows forward traversal
• Confusing pointer count with memory usage

2. Which of the following is the correct Rust struct definition for a doubly linked list node holding an integer value?

easy

A. struct Node { value: i32, next: Option, prev: Option<*mut Node> }

B. struct Node { value: i32, next: Option, prev: Option }

C. struct Node { value: i32, next: Option, prev: Option }

D. struct Node { value: i32, next: Option<Node>, prev: Option<Node> }

Solution

1. Step 1: Understand ownership and pointers in Rust doubly linked list

   Rust's ownership rules prevent two owning pointers to the same data. The next pointer can own the next node using Box, but the prev pointer must be a raw pointer or weak reference to avoid ownership conflicts.

2. Step 2: Analyze options for prev pointer

   struct Node { value: i32, next: Option, prev: Option<*mut Node> } uses Option<*mut Node> (a raw mutable pointer) for prev, which is common in unsafe doubly linked list implementations. struct Node { value: i32, next: Option, prev: Option } incorrectly uses Box for prev, causing double ownership. struct Node { value: i32, next: Option, prev: Option } uses Rc which is for shared ownership but not mutable by default. struct Node { value: i32, next: Option<Node>, prev: Option<Node> } uses Option<Node> which is invalid because Node is not sized.

3. Final Answer:

   struct Node { value: i32, next: Option, prev: Option<*mut Node> } -> Option A

4. Quick Check:

   Prev pointer = raw pointer to avoid ownership [OK]

Hint: Prev pointer often uses raw pointer in Rust doubly linked lists [OK]

Common Mistakes:

• Using Box for both next and prev causing ownership errors
• Using Rc without mutability for prev
• Trying to store Node directly without Box or pointer

3. Given the following Rust code snippet for a doubly linked list node insertion, what will be the printed output?

struct Node {
    value: i32,
    next: Option,
    prev: Option<*mut Node>,
}

fn main() {
    let mut first = Box::new(Node { value: 1, next: None, prev: None });
    let mut second = Box::new(Node { value: 2, next: None, prev: None });

    second.prev = Some(&mut *first as *mut _);
    first.next = Some(second);

    println!("{} -> {}", first.value, first.next.as_ref().unwrap().value);
}

medium

A. Runtime panic due to null pointer

B. 2 -> 1

C. Compilation error due to borrowing

D. 1 -> 2

Solution

1. Step 1: Understand node creation and linking

   Two nodes are created: first with value 1, second with value 2. The second node's prev pointer is set to point to first. The first node's next pointer is set to own second.

2. Step 2: Analyze the print statement

   The print accesses first.value (1) and first.next's value (2). Since first.next is Some(second), unwrap() succeeds and prints "1 -> 2".

3. Final Answer:

   1 -> 2 -> Option D

4. Quick Check:

   First node value then next node value = 1 -> 2 [OK]

Hint: Print first node then unwrap next for correct order [OK]

Common Mistakes:

• Confusing prev pointer direction
• Expecting compilation error due to unsafe pointer
• Assuming runtime panic without dereferencing prev

4. Identify the error in this Rust doubly linked list node struct and fix it:

struct Node {
    value: i32,
    next: Option,
    prev: Option,
}

medium

A. Change next to Option for shared ownership

B. Remove prev pointer completely

C. Change prev to Option<*mut Node> to avoid double ownership

D. Use Option<Node> for both next and prev

Solution

1. Step 1: Identify ownership problem with prev pointer

   Using Box for both next and prev means both own the node, causing double ownership which Rust disallows.

2. Step 2: Fix by changing prev pointer type

   Changing prev to Option<*mut Node> uses a raw pointer, which does not own the node and avoids ownership conflicts.

3. Final Answer:

   Change prev to Option<*mut Node> to avoid double ownership -> Option C

4. Quick Check:

   Prev pointer must not own node to prevent ownership error [OK]

Hint: Prev pointer uses raw pointer to avoid ownership conflicts [OK]

Common Mistakes:

• Using Box for prev causing compile errors
• Trying to remove prev pointer losing backward traversal
• Using Option<Node> which is invalid due to size

5. You want to implement a safe doubly linked list in Rust that allows multiple owners of nodes and mutable access. Which combination of pointer types is the best design choice?

hard

A. Use raw pointers (*mut Node) for both next and prev

B. Use Rc for next and Weak for prev

C. Use Option<Node> for next and Option<Node> for prev

D. Use Box<Node> for next and Box<Node> for prev

Solution

1. Step 1: Understand ownership and mutability requirements

   Multiple owners require reference counting (Rc). Mutable access requires interior mutability (RefCell).

2. Step 2: Choose pointer types for next and prev

   Next pointer uses Rc to own and allow mutation. Prev pointer uses Weak to avoid strong reference cycles and memory leaks.

3. Final Answer:

   Use Rc for next and Weak for prev -> Option B

4. Quick Check:

   Rc + RefCell + Weak = safe mutable doubly linked list [OK]

Hint: Use Rc + RefCell for ownership and Weak for back links [OK]

Common Mistakes:

• Using Box for both pointers causing ownership errors
• Using raw pointers risking unsafe code
• Ignoring Weak to prevent reference cycles