Why Doubly Linked List Over Singly Linked List in DSA Rust - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a doubly linked list node with a previous pointer.

DSA Rust

struct Node {
    value: i32,
    next: Option<Box<Node>>,
    [1]: Option<Box<Node>>,
}

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

Aprev

Bprevious_node

Cprev_node

Dprevious

Attempts:

3 left

💡 Hint

Common Mistakes

Using longer or inconsistent names like 'previous_node' which are less common.

Confusing the previous pointer with the next pointer.

The correct field name for the previous pointer in a doubly linked list node is prev. This is a common convention and keeps the code clear and concise.

2fill in blank

medium

Complete the code to move backward in a doubly linked list using the previous pointer.

DSA Rust

fn move_backward(node: &Option<Box<Node>>) -> Option<&Box<Node>> {
    node.as_ref()?.[1]
}

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

Anext

Bprevious

Cprev

Dprev.as_ref()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'prev' to move backward.

Not calling 'as_ref()' on the Option, causing type errors.

To move backward in a doubly linked list, we use the prev pointer. Since prev is an Option, we call as_ref() to get a reference to the boxed node.

3fill in blank

hard

Fix the error in the code to correctly insert a new node after the current node in a doubly linked list.

DSA Rust

fn insert_after(current: &mut Box<Node>, value: i32) {
    let mut new_node = Box::new(Node {
        value,
        next: current.next.take(),
        [1]: Some(current.clone()),
    });
    current.next = Some(new_node);
}

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

Aprevious

Bprev

Cprev.clone()

Dprev.as_ref()

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect field names like 'previous' or calling methods on 'prev' incorrectly.

Not cloning the current node when assigning to prev.

The new node's previous pointer should be set to prev field pointing to the current node. Using prev is the correct field name.

4fill in blank

hard

Fill both blanks to correctly update pointers when removing a node from a doubly linked list.

DSA Rust

fn remove_node(node: &mut Box<Node>) {
    if let Some(mut next_node) = node.next.take() {
        next_node.[1] = node.[2].take();
    }
}

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

Aprev

Bnext

Cprev.as_mut()

Dprev.clone()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'prev' for updating backward links.

Not taking ownership properly causing borrow errors.

When removing a node, the next node's previous pointer should be updated to the current node's previous pointer. Both blanks should be filled with prev.

5fill in blank

hard

Fill all three blanks to create a map of node values to their previous node values in a doubly linked list.

DSA Rust

let map = nodes.iter().map(|node| {
    (node.value, node.[1].as_ref().map(|n| n.[2]).unwrap_or([3]))
}).collect::<std::collections::HashMap<_, _>>();

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

Aprev

Bvalue

C0

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'prev' to get previous nodes.

Not handling the Option properly causing runtime errors.

To map each node's value to its previous node's value, use the prev pointer, then access the value field of that node. If there is no previous node, use 0 as default.

Practice

1. Why might a doubly linked list be preferred over a singly linked list in some cases?

easy

A. Because it cannot be used to implement stacks

B. Because it allows traversal in both forward and backward directions

C. Because it only stores one pointer per node

D. Because it uses less memory than singly linked list

Solution

1. Step 1: Understand traversal capabilities

   A singly linked list allows traversal only in one direction, from head to tail.

2. Step 2: Compare with doubly linked list

   A doubly linked list stores two pointers per node, allowing traversal both forward and backward.

3. Final Answer:

   Because it allows traversal in both forward and backward directions -> Option B

4. Quick Check:

   Traversal direction = Both forward and backward [OK]

Hint: Doubly linked lists have two pointers per node [OK]

Common Mistakes:

• Thinking doubly linked lists use less memory
• Confusing pointer count per node
• Assuming singly linked lists allow backward traversal

2. Which of the following is the correct way to declare a node struct for a doubly linked list in Rust?

easy

A. struct Node { data: i32, prev: Option, next: Option }

B. struct Node { data: i32, prev: Box<Node> }

C. struct Node { data: i32, next: Option }

D. struct Node { data: i32, next: Node }

Solution

1. Step 1: Identify doubly linked list node fields

   A doubly linked list node must have pointers to both previous and next nodes, usually wrapped in Option and Box for ownership.

2. Step 2: Check each option

   struct Node { data: i32, prev: Option, next: Option } correctly declares both prev and next as Option. Others miss prev or use incorrect types.

3. Final Answer:

   struct Node { data: i32, prev: Option, next: Option } -> Option A

4. Quick Check:

   Prev and next pointers = struct Node { data: i32, prev: Option, next: Option } [OK]

Hint: Doubly linked nodes have prev and next pointers [OK]

Common Mistakes:

• Omitting prev pointer
• Using Box<Node> without Option
• Using next as Node instead of pointer

3. Given this Rust code snippet for a doubly linked list insertion at the end, what will be the printed list after inserting 3 nodes with values 10, 20, 30?

let mut list = DoublyLinkedList::new();
list.push_back(10);
list.push_back(20);
list.push_back(30);
list.print_forward();

medium

A. 10 -> 30 -> 20 -> null

B. 30 -> 20 -> 10 -> null

C. null -> 10 -> 20 -> 30

D. 10 -> 20 -> 30 -> null

Solution

1. Step 1: Understand push_back behavior

   push_back adds nodes at the end, so nodes are appended in order 10, then 20, then 30.

2. Step 2: Trace print_forward output

   Printing forward shows nodes from head to tail: 10 -> 20 -> 30 -> null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option D

4. Quick Check:

   Insertion order = Printed order [OK]

Hint: push_back appends at tail, print_forward shows head to tail [OK]

Common Mistakes:

• Confusing push_back with push_front
• Printing backward instead of forward
• Mixing node order in output

4. In this Rust doubly linked list code, what is the bug causing incorrect backward traversal?

fn print_backward(&self) {
    let mut current = self.tail.clone();
    while let Some(node) = current {
        print!("{} -> ", node.data);
        current = node.next.clone();
    }
    println!("null");
}

medium

A. Not cloning the tail pointer

B. Printing node.data instead of node.prev

C. Using node.next instead of node.prev for backward traversal

D. Missing a break statement in the loop

Solution

1. Step 1: Analyze traversal direction

   Backward traversal should follow prev pointers, but code uses node.next which goes forward.

2. Step 2: Identify correct pointer usage

   Replacing node.next with node.prev fixes backward traversal.

3. Final Answer:

   Using node.next instead of node.prev for backward traversal -> Option C

4. Quick Check:

   Backward traversal = Use prev pointer [OK]

Hint: Backward traversal uses prev pointer, not next [OK]

Common Mistakes:

• Confusing next and prev pointers
• Not cloning pointers properly
• Printing wrong data fields

5. You want to implement a feature that allows quick removal of a node from the middle of a list without traversing from the head. Why is a doubly linked list better than a singly linked list for this task?

hard

A. Because doubly linked list nodes have pointers to previous nodes, enabling direct access to neighbors

B. Because singly linked lists store extra data for quick removal

C. Because doubly linked lists use less memory, making removal faster

D. Because singly linked lists allow backward traversal

Solution

1. Step 1: Understand node removal requirements

   Removing a node requires updating pointers of its neighbors, including the previous node.

2. Step 2: Compare singly and doubly linked lists

   Singly linked lists lack backward pointers, so to remove a node, you must traverse from head to find the previous node. Doubly linked lists have prev pointers, allowing direct access to neighbors.

3. Final Answer:

   Because doubly linked list nodes have pointers to previous nodes, enabling direct access to neighbors -> Option A

4. Quick Check:

   Prev pointer enables quick removal [OK]

Hint: Prev pointer lets you remove nodes without full traversal [OK]

Common Mistakes:

• Assuming singly linked lists have backward pointers
• Thinking memory usage affects removal speed
• Confusing traversal direction capabilities