Insert at End of Doubly Linked List in DSA Rust - Build from Scratch

Insert at End of Doubly Linked List

📖 Scenario: You are building a simple playlist manager. Each song is a node in a doubly linked list. You want to add new songs to the end of the playlist.

🎯 Goal: Create a doubly linked list and write code to insert a new node at the end. Finally, print the playlist from start to end.

📋 What You'll Learn

Define a struct Node with data, prev, and next fields

Create a struct DoublyLinkedList with head and tail pointers

Implement a method insert_at_end to add a new node at the end

Print the list from head to tail showing all node data

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, undo-redo features, and navigation history where you need to move forward and backward easily.

💼 Career

Understanding linked lists is fundamental for software engineers working on data structures, memory management, and performance optimization.

1

Create the Node and DoublyLinkedList structs

Define a struct called Node with a field data of type i32, and two Option<Box<Node>> fields called prev and next. Then define a struct called DoublyLinkedList with two fields: head and tail, both of type Option<Box<Node>>.

DSA Rust

# Define Node and DoublyLinkedList structs
# Your code here

Hint

Use Option<Box<Node>> for prev and next to allow null pointers.

2

Add a new method insert_at_end to DoublyLinkedList

Inside DoublyLinkedList, add a method called insert_at_end that takes &mut self and a parameter data of type i32. This method will add a new node with the given data at the end of the list.

DSA Rust

impl DoublyLinkedList {
    // Add insert_at_end method here
    # Your code here
}

Hint

Use self.tail.take() to get the current tail, then update pointers to add the new node at the end.

3

Initialize an empty DoublyLinkedList and insert nodes

Create a mutable variable called list of type DoublyLinkedList with head and tail set to None. Then call insert_at_end on list to insert nodes with data 10, 20, and 30.

DSA Rust

fn main() {
    // Create empty list
    // Insert 10, 20, 30 at end
    # Your code here
}

Hint

Initialize list with head and tail as None. Then call insert_at_end three times with the given values.

4

Print the list from head to tail

Add code inside main to print the data of each node from head to tail in the format: 10 -> 20 -> 30 -> None. Use a variable current to traverse the list starting from list.head.as_ref().

DSA Rust

fn main() {
    let mut list = DoublyLinkedList { head: None, tail: None };
    list.insert_at_end(10);
    list.insert_at_end(20);
    list.insert_at_end(30);

    // Traverse and print the list
    # Your code here
}

Hint

Use a while let Some(node) = current loop to traverse and print each node's data followed by ->. End with None.

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