Insert at Specific Position in Doubly Linked List in DSA Rust - Build from Scratch

Insert at Specific Position in Doubly Linked List

📖 Scenario: You are managing a playlist of songs. Each song is linked to the previous and next song, allowing you to move forward or backward easily. Sometimes, you want to add a new song at a specific spot in the playlist.

🎯 Goal: Build a doubly linked list of songs and insert a new song at a given position.

📋 What You'll Learn

Create a doubly linked list with three songs: "Song A", "Song B", and "Song C"

Create a variable called position to hold the position where the new song will be inserted

Write a function called insert_at_position that inserts a new song at the given position in the doubly linked list

Print the playlist from head to tail after insertion, showing song titles separated by -> and ending with null

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps where you need to move forward and backward through items easily.

💼 Career

Understanding linked lists and pointer manipulation is important for software engineers working on low-level data structures, performance-critical applications, or systems programming.

1

Create the initial doubly linked list

Create a doubly linked list with three nodes containing the song titles "Song A", "Song B", and "Song C". Use a struct called Node with fields data (String), prev (Option<Box<Node>>), and next (Option<Box<Node>>). Create a variable called head that points to the first node.

DSA Rust

// Define Node struct and create the doubly linked list with three songs
// Your code here

Hint

Use Rc and RefCell to allow multiple ownership and mutability of nodes.

2

Create the position variable

Create a variable called position and set it to 2. This will be the position where the new song will be inserted (positions start at 1).

DSA Rust

use std::rc::Rc;
use std::cell::RefCell;

#[derive(Debug)]
struct Node {
    data: String,
    prev: Option>>,
    next: Option>>,
}

fn main() {
    let node1 = Rc::new(RefCell::new(Node { data: "Song A".to_string(), prev: None, next: None }));
    let node2 = Rc::new(RefCell::new(Node { data: "Song B".to_string(), prev: None, next: None }));
    let node3 = Rc::new(RefCell::new(Node { data: "Song C".to_string(), prev: None, next: None }));

    node1.borrow_mut().next = Some(Rc::clone(&node2));
    node2.borrow_mut().prev = Some(Rc::clone(&node1));
    node2.borrow_mut().next = Some(Rc::clone(&node3));
    node3.borrow_mut().prev = Some(Rc::clone(&node2));

    let head = Some(Rc::clone(&node1));

    // Create a variable called position and set it to 2
    // Your code here
}

Hint

Just create a variable named position and assign it the value 2.

3

Write the insert_at_position function

Write a function called insert_at_position that takes head: Option<Rc<RefCell<Node>>>, data: String, and position: usize. It inserts a new node with the given data at the given position in the doubly linked list and returns the updated head. Use Rc and RefCell to manage nodes. Positions start at 1. If position is 1, insert at the head. Otherwise, insert at the correct place in the list.

DSA Rust

use std::rc::Rc;
use std::cell::RefCell;

#[derive(Debug)]
struct Node {
    data: String,
    prev: Option>>,
    next: Option>>,
}

fn main() {
    let node1 = Rc::new(RefCell::new(Node { data: "Song A".to_string(), prev: None, next: None }));
    let node2 = Rc::new(RefCell::new(Node { data: "Song B".to_string(), prev: None, next: None }));
    let node3 = Rc::new(RefCell::new(Node { data: "Song C".to_string(), prev: None, next: None }));

    node1.borrow_mut().next = Some(Rc::clone(&node2));
    node2.borrow_mut().prev = Some(Rc::clone(&node1));
    node2.borrow_mut().next = Some(Rc::clone(&node3));
    node3.borrow_mut().prev = Some(Rc::clone(&node2));

    let head = Some(Rc::clone(&node1));

    let position = 2;

    // Write the insert_at_position function below
    // Your code here
}

Hint

Traverse the list until the node before the desired position. Then update the prev and next pointers of the new node and its neighbors carefully.

4

Print the playlist after insertion

Print the playlist from head to tail after insertion. Use a variable called current to traverse from head. Print each song's data followed by -> . End the print with null.

DSA Rust

use std::rc::Rc;
use std::cell::RefCell;

#[derive(Debug)]
struct Node {
    data: String,
    prev: Option>>,
    next: Option>>,
}

fn insert_at_position(head: Option>>, data: String, position: usize) -> Option>> {
    let new_node = Rc::new(RefCell::new(Node { data, prev: None, next: None }));

    if position == 1 {
        if let Some(old_head) = head {
            new_node.borrow_mut().next = Some(Rc::clone(&old_head));
            old_head.borrow_mut().prev = Some(Rc::clone(&new_node));
        }
        return Some(new_node);
    }

    let mut current = head.clone();
    let mut current_position = 1;

    while let Some(node) = current.clone() {
        if current_position + 1 == position {
            let next_node = node.borrow().next.clone();
            new_node.borrow_mut().prev = Some(Rc::clone(&node));
            new_node.borrow_mut().next = next_node.clone();
            node.borrow_mut().next = Some(Rc::clone(&new_node));
            if let Some(next) = next_node {
                next.borrow_mut().prev = Some(Rc::clone(&new_node));
            }
            break;
        }
        current = node.borrow().next.clone();
        current_position += 1;
    }

    head
}

fn main() {
    let node1 = Rc::new(RefCell::new(Node { data: "Song A".to_string(), prev: None, next: None }));
    let node2 = Rc::new(RefCell::new(Node { data: "Song B".to_string(), prev: None, next: None }));
    let node3 = Rc::new(RefCell::new(Node { data: "Song C".to_string(), prev: None, next: None }));

    node1.borrow_mut().next = Some(Rc::clone(&node2));
    node2.borrow_mut().prev = Some(Rc::clone(&node1));
    node2.borrow_mut().next = Some(Rc::clone(&node3));
    node3.borrow_mut().prev = Some(Rc::clone(&node2));

    let head = Some(Rc::clone(&node1));

    let position = 2;

    let head = insert_at_position(head, "Song X".to_string(), position);

    // Print the playlist from head to tail
    // Your code here
}

Hint

Use a while let Some(node) = current loop to traverse from head to tail. Print each node's data followed by -> . After the loop, print null.

Practice

1. What does inserting at a specific position in a doubly linked list involve?

easy

A. Adjusting the previous and next pointers of nodes around the position

B. Only changing the head pointer of the list

C. Reversing the entire list before insertion

D. Deleting the node at that position first

Solution

1. Step 1: Understand doubly linked list node connections

   Each node has pointers to both previous and next nodes, so insertion requires updating these pointers around the insertion point.

2. Step 2: Adjust pointers for insertion

   To insert at a specific position, update the previous node's next pointer and the next node's previous pointer to point to the new node.

3. Final Answer:

   Adjusting the previous and next pointers of nodes around the position -> Option A

4. Quick Check:

   Pointer updates = Adjust previous and next pointers [OK]

Hint: Remember: update both previous and next pointers [OK]

Common Mistakes:

• Only updating one pointer and ignoring the other
• Not handling edge cases like inserting at head or tail
• Forgetting to update the new node's pointers

2. Which Rust code snippet correctly declares a doubly linked list node struct for insertion?

easy

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

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

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

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

Solution

1. Step 1: Understand Rust ownership for doubly linked lists

   Doubly linked lists require shared ownership and mutability, so Rc is used to allow multiple owners and interior mutability.

2. Step 2: Identify correct pointer types

   Option> allows optional pointers with shared ownership and mutability, suitable for prev and next pointers.

3. Final Answer:

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

4. Quick Check:

   Use Rc<RefCell> for shared mutable pointers [OK]

Hint: Use Rc<RefCell> for doubly linked list nodes in Rust [OK]

Common Mistakes:

• Using Box without Rc causes ownership errors
• Using raw pointers without unsafe code
• Not wrapping pointers in Option for nullable links

3. Given this Rust code snippet inserting a node at position 2 in a doubly linked list with nodes 1 <-> 3, what is the list after insertion of node 2?

medium

A. 3 <-> 2 <-> 1

B. 2 <-> 1 <-> 3

C. 1 <-> 2 <-> 3

D. 1 <-> 3 <-> 2

Solution

1. Step 1: Identify insertion position and existing list

   The list is 1 <-> 3, and we insert node 2 at position 2, meaning after node 1 and before node 3.

2. Step 2: Insert node 2 between nodes 1 and 3

   Update node 1's next to node 2, node 2's prev to node 1, node 2's next to node 3, and node 3's prev to node 2.

3. Final Answer:

   1 <-> 2 <-> 3 -> Option C

4. Quick Check:

   Insert at position 2 places node 2 between 1 and 3 [OK]

Hint: Insert node between previous and next nodes at position [OK]

Common Mistakes:

• Inserting node at wrong position
• Not updating both prev and next pointers
• Confusing position indexing starting at 0 or 1

4. In this Rust code for inserting at position 3, the program panics with 'index out of bounds'. What is the likely cause?

medium

A. The new node's pointers are not initialized

B. The list has fewer than 2 nodes, so position 3 is invalid

C. The head pointer is null

D. The insertion code forgets to update the tail pointer

Solution

1. Step 1: Analyze the error message and position

   The panic 'index out of bounds' means the code tried to access a position beyond the list length. Position 3 requires at least 2 nodes before it.

2. Step 2: Check list length against insertion position

   If the list has fewer than 2 nodes, position 3 is invalid, causing the panic.

3. Final Answer:

   The list has fewer than 2 nodes, so position 3 is invalid -> Option B

4. Quick Check:

   Position > list length causes index out of bounds [OK]

Hint: Check list length before inserting at position [OK]

Common Mistakes:

• Not validating position against list length
• Assuming tail pointer updates fix index errors
• Ignoring null or empty list cases

5. You have a doubly linked list with nodes 10 <-> 20 <-> 30. Write the steps to insert a node with value 25 at position 3 and show the final list.

hard

A. 25 <-> 10 <-> 20 <-> 30

B. 10 <-> 25 <-> 20 <-> 30

C. 10 <-> 20 <-> 30 <-> 25

D. 10 <-> 20 <-> 25 <-> 30

Solution

1. Step 1: Locate position 3 in the list

   Position 3 means the new node goes after the second node (20) and before the third node (30).

2. Step 2: Update pointers to insert node 25

   Set node 20's next to new node 25, new node 25's prev to node 20, new node 25's next to node 30, and node 30's prev to new node 25.

3. Final Answer:

   10 <-> 20 <-> 25 <-> 30 -> Option D

4. Quick Check:

   Insert at position 3 places 25 between 20 and 30 [OK]

Hint: Insert between nodes at position - 1 and position [OK]

Common Mistakes:

• Inserting node at wrong position
• Not updating both prev and next pointers correctly
• Confusing position indexing starting at 0 or 1