Insert at End of Doubly Linked List in DSA Rust - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting 4 at the end?

Consider a doubly linked list initially containing 1 <-> 2 <-> 3. We insert 4 at the end. What is the printed list?

DSA Rust

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

struct DoublyLinkedList {
    head: Option<Box<Node>>,
}

impl DoublyLinkedList {
    fn new() -> Self {
        DoublyLinkedList { head: None }
    }

    fn insert_end(&mut self, data: i32) {
        let mut new_node = Box::new(Node { data, prev: None, next: None });
        if self.head.is_none() {
            self.head = Some(new_node);
            return;
        }
        let mut current = self.head.as_mut().unwrap();
        while current.next.is_some() {
            current = current.next.as_mut().unwrap();
        }
        new_node.prev = Some(&mut **current as *mut Node);
        current.next = Some(new_node);
    }

    fn print_list(&self) {
        let mut current = self.head.as_ref();
        while let Some(node) = current {
            print!("{} <-> ", node.data);
            current = node.next.as_ref();
        }
        println!("null");
    }
}

fn main() {
    let mut dll = DoublyLinkedList::new();
    dll.insert_end(1);
    dll.insert_end(2);
    dll.insert_end(3);
    dll.insert_end(4);
    dll.print_list();
}

A 1 <-> 2 <-> 3 <-> null

B 1 <-> 2 <-> 3 <-> 4 <-> null

C 4 <-> 1 <-> 2 <-> 3 <-> null

D 1 <-> 2 <-> 3 <-> 4

💡 Hint

Trace the insert_end function and how it adds the new node at the end.

The insert_end method traverses to the last node and adds the new node after it. The print_list method prints all nodes ending with null.

❓ Predict Output

intermediate

What is the output after inserting 10 at the end of an empty list?

Start with an empty doubly linked list. Insert 10 at the end. What is the printed list?

DSA Rust

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

struct DoublyLinkedList {
    head: Option<Box<Node>>,
}

impl DoublyLinkedList {
    fn new() -> Self {
        DoublyLinkedList { head: None }
    }

    fn insert_end(&mut self, data: i32) {
        let new_node = Box::new(Node { data, prev: None, next: None });
        if self.head.is_none() {
            self.head = Some(new_node);
            return;
        }
        let mut current = self.head.as_mut().unwrap();
        while current.next.is_some() {
            current = current.next.as_mut().unwrap();
        }
        new_node.prev = Some(&mut **current as *mut Node);
        current.next = Some(new_node);
    }

    fn print_list(&self) {
        let mut current = self.head.as_ref();
        while let Some(node) = current {
            print!("{} <-> ", node.data);
            current = node.next.as_ref();
        }
        println!("null");
    }
}

fn main() {
    let mut dll = DoublyLinkedList::new();
    dll.insert_end(10);
    dll.print_list();
}

A null

B null <-> 10

C 10

D 10 <-> null

💡 Hint

Check how insert_end handles empty list case.

When the list is empty, insert_end sets head to the new node. Printing shows '10 <-> null'.

🔧 Debug

advanced

What error occurs when running this insert_end code?

This Rust code tries to insert at the end of a doubly linked list. What error does it produce?

DSA Rust

fn insert_end(&mut self, data: i32) {
    let mut new_node = Box::new(Node { data, prev: None, next: None });
    if self.head.is_none() {
        self.head = Some(new_node);
        return;
    }
    let mut current = self.head.as_mut().unwrap();
    while current.next.is_some() {
        current = current.next.as_mut().unwrap();
    }
    new_node.prev = Some(Box::new(Node { data: current.data, prev: None, next: None }));
    current.next = Some(new_node);
}

A Compile-time error: cannot move out of borrowed content

B No error, runs correctly

C Runtime panic: index out of bounds

D Compile-time error: mismatched types

💡 Hint

Look at how prev is assigned with Box::new(Node...) inside insert_end.

The code tries to move out of a borrowed reference when assigning prev, causing a compile-time error.

❓ Predict Output

advanced

What is the output after inserting 5 at the end twice?

Start with a doubly linked list containing 1 <-> 3. Insert 5 at the end two times. What is the printed list?

DSA Rust

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

struct DoublyLinkedList {
    head: Option<Box<Node>>,
}

impl DoublyLinkedList {
    fn new() -> Self {
        DoublyLinkedList { head: None }
    }

    fn insert_end(&mut self, data: i32) {
        let new_node = Box::new(Node { data, prev: None, next: None });
        if self.head.is_none() {
            self.head = Some(new_node);
            return;
        }
        let mut current = self.head.as_mut().unwrap();
        while current.next.is_some() {
            current = current.next.as_mut().unwrap();
        }
        new_node.prev = Some(&mut **current as *mut Node);
        current.next = Some(new_node);
    }

    fn print_list(&self) {
        let mut current = self.head.as_ref();
        while let Some(node) = current {
            print!("{} <-> ", node.data);
            current = node.next.as_ref();
        }
        println!("null");
    }
}

fn main() {
    let mut dll = DoublyLinkedList::new();
    dll.insert_end(1);
    dll.insert_end(3);
    dll.insert_end(5);
    dll.insert_end(5);
    dll.print_list();
}

A 1 <-> 3 <-> 5 <-> 5 <-> null

B 1 <-> 3 <-> 5 <-> null

C 1 <-> 3 <-> null

D 5 <-> 5 <-> 1 <-> 3 <-> null

💡 Hint

Check how insert_end adds nodes sequentially at the end.

Each insert_end adds a new node at the end. After two insertions of 5, the list ends with two 5 nodes.

🧠 Conceptual

expert

Why is using Box for prev problematic in a doubly linked list in Rust?

In Rust, a doubly linked list node has fields: data, prev: Option>, next: Option>. Why is this design problematic?

A Because prev should be a raw pointer to avoid borrow checker errors

B Because Box<Node> cannot store integers

C Because Box<Node> for prev creates ownership cycles leading to memory leaks

D Because Box<Node> does not allow mutable access

💡 Hint

Think about ownership and cycles in Rust's memory model.

Using Box for prev creates strong ownership cycles between nodes, preventing memory from being freed.

Practice

1. What is the main purpose of inserting a node at the end of a doubly linked list?

easy

A. To reverse the list

B. To remove the first node

C. To add a new element after the last node

D. To find the middle node

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes connected forward and backward, allowing traversal in both directions.

2. Step 2: Purpose of insertion at the end

   Inserting at the end means adding a new node after the current last node, updating pointers accordingly.

3. Final Answer:

   To add a new element after the last node -> Option C

4. Quick Check:

   Insert at end = add after last node [OK]

Hint: Insert at end means add after last node [OK]

Common Mistakes:

• Confusing insertion at end with deletion
• Thinking insertion reverses the list
• Assuming insertion happens at the start

2. Which Rust code snippet correctly creates a new node and inserts it at the end of a doubly linked list?

easy

A. let new_node = Node::new(value); new_node.prev = Some(head); head.next = Some(new_node);

B. let new_node = Node::new(value); head.prev = Some(new_node); new_node.next = Some(head);

C. let new_node = Node::new(value); new_node.next = Some(tail); tail.prev = Some(new_node);

D. let new_node = Node::new(value); tail.next = Some(new_node); new_node.prev = Some(tail);

Solution

1. Step 1: Create new node with given value

   Use Node::new(value) to create a new node.

2. Step 2: Link new node after tail

   Set tail.next to new_node and new_node.prev to tail to insert at end.

3. Final Answer:

   let new_node = Node::new(value); tail.next = Some(new_node); new_node.prev = Some(tail); -> Option D

4. Quick Check:

   New node after tail = let new_node = Node::new(value); tail.next = Some(new_node); new_node.prev = Some(tail); [OK]

Hint: New node's prev points to tail; tail's next points to new node [OK]

Common Mistakes:

• Linking new node before head instead of after tail
• Swapping next and prev pointers incorrectly
• Not updating tail's next pointer

3. Given the Rust code below, what will be the printed list after inserting 4 at the end?

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

// Assume list initially: 1 <-> 2 <-> 3
// Insert 4 at end
// Print list from head to tail

medium

A. 1 <-> 2 <-> 3 <-> 4 <-> null

B. 4 <-> 1 <-> 2 <-> 3 <-> null

C. 1 <-> 2 <-> 4 <-> 3 <-> null

D. 1 <-> 2 <-> 3 <-> null

Solution

1. Step 1: Understand initial list structure

   The list is 1 linked to 2, linked to 3, with 3 as tail.

2. Step 2: Insert 4 at the end

   Adding 4 after 3 updates 3.next to 4 and 4.prev to 3.

3. Final Answer:

   1 <-> 2 <-> 3 <-> 4 <-> null -> Option A

4. Quick Check:

   Insert at end adds 4 after 3 [OK]

Hint: New node appears after last node in printed list [OK]

Common Mistakes:

• Inserting new node at start instead of end
• Not updating next pointer of last node
• Printing list without new node

4. Identify the error in this Rust code snippet for inserting at the end of a doubly linked list:

fn insert_end(tail: &mut Option, value: i32) {
    let mut new_node = Box::new(Node::new(value));
    new_node.prev = tail.clone();
    tail = Some(new_node);
}

medium

A. tail is not updated correctly; should update tail's next pointer

B. new_node.prev should be None for end insertion

C. new_node should not be boxed

D. value should be mutable

Solution

1. Step 1: Check pointer updates

   tail is reassigned locally, but tail's next pointer is not updated to link new_node.

2. Step 2: Correct linking needed

   Must set tail's next to new_node and new_node.prev to tail, then update tail reference.

3. Final Answer:

   tail is not updated correctly; should update tail's next pointer -> Option A

4. Quick Check:

   Update tail.next to new_node [OK]

Hint: Always update tail's next pointer when inserting at end [OK]

Common Mistakes:

• Only reassigning tail without linking nodes
• Setting prev pointer incorrectly
• Ignoring ownership rules in Rust

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30. You want to insert 25 at the end but also keep a pointer to the middle node for quick access. After insertion, which node should the middle pointer reference to keep it as the middle of the list?

hard

A. 25

B. 20

C. 30

D. 10

Solution

1. Step 1: Understand list before insertion

   List is 10 <-> 20 <-> 30; middle is 20 (second of three nodes).

2. Step 2: Insert 25 at end and find new middle

   List becomes 10 <-> 20 <-> 30 <-> 25. With 4 nodes, middle is between 20 and 30, usually the second node (20) to keep balance.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   Middle of 4 nodes = 2nd node [OK]

Hint: Middle of even nodes is lower middle [OK]

Common Mistakes:

• Moving middle pointer to last inserted node
• Choosing last node as middle
• Not updating middle pointer after insertion