Detect if a Linked List is Circular in DSA Rust - Build from Scratch

Detect if a Linked List is Circular

📖 Scenario: Imagine you have a chain of connected train cars. Sometimes, the last car connects back to an earlier car, making a loop. We want to check if the train cars form such a loop or if the chain ends normally.

🎯 Goal: Build a simple linked list in Rust and write code to check if the list is circular (has a loop) or not.

📋 What You'll Learn

Create a linked list with exactly 4 nodes with values 10, 20, 30, and 40

Add a boolean variable called make_circular to decide if the list should loop back

Write a function called is_circular that returns true if the list has a loop, otherwise false

Print the result of is_circular for the created list

💡 Why This Matters

🌍 Real World

Circular linked lists are used in real-world applications like music playlists, task scheduling, and multiplayer games where the sequence repeats.

💼 Career

Understanding how to detect loops in linked lists is important for software engineers working on data structures, memory management, and debugging complex pointer-based code.

1

Create a linked list with 4 nodes

Create a linked list with 4 nodes having values 10, 20, 30, and 40. Use a struct called Node with fields value: i32 and next: Option>. Create a variable called head pointing to the first node.

DSA Rust

// Define Node struct and create linked list with 4 nodes
// Your code here

Hint

Define the Node struct first. Then create nodes from last to first, linking each with next: Some(Box::new(...)).

2

Add a boolean to control circular linking

Add a boolean variable called make_circular and set it to true. This will decide if the last node links back to the head node to form a loop.

DSA Rust

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

fn main() {
    // Add make_circular variable here
    // Your code here

    let node4 = Node { value: 40, next: None };
    let node3 = Node { value: 30, next: Some(Box::new(node4)) };
    let node2 = Node { value: 20, next: Some(Box::new(node3)) };
    let head = Node { value: 10, next: Some(Box::new(node2)) };
}

Hint

Just add let make_circular = true; inside main.

3

Write a function to detect if the list is circular

Write a function called is_circular that takes a reference to a Node and returns true if the list has a loop, otherwise false. Use two pointers called slow and fast to move through the list. If they meet, return true. If fast or fast.next is None, return false.

DSA Rust

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

fn is_circular(head: &Node) -> bool {
    // Your code here
}

fn main() {
    let make_circular = true;

    let node4 = Node { value: 40, next: None };
    let node3 = Node { value: 30, next: Some(Box::new(node4)) };
    let node2 = Node { value: 20, next: Some(Box::new(node3)) };
    let head = Node { value: 10, next: Some(Box::new(node2)) };
}

Hint

Use two pointers moving at different speeds. If they meet, the list is circular.

4

Make the list circular if requested and print the result

If make_circular is true, link the last node's next to the head to form a loop. Then call is_circular(&head) and print the result using println!("{}", result).

DSA Rust

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

fn is_circular(head: &Node) -> bool {
    let mut slow = Some(head);
    let mut fast = Some(head);

    while let (Some(s), Some(f)) = (slow, fast) {
        slow = s.next.as_deref();
        fast = f.next.as_deref().and_then(|n| n.next.as_deref());

        if slow.is_some() && slow == fast {
            return true;
        }
    }
    false
}

fn main() {
    let make_circular = true;

    // Your code here
}

Hint

To form a loop, link the last node's next to the head. Then call is_circular(&head) and print the result.

Practice

1. What does it mean if a linked list is circular?

easy

A. The linked list has no nodes

B. The linked list contains only one node

C. The linked list nodes are sorted in ascending order

D. The last node points back to one of the previous nodes, forming a loop

Solution

1. Step 1: Understand linked list structure

   A linked list is a chain of nodes where each node points to the next node.

2. Step 2: Define circular linked list

   If the last node points back to any previous node, it creates a loop, making the list circular.

3. Final Answer:

   The last node points back to one of the previous nodes, forming a loop -> Option D

4. Quick Check:

   Circular linked list = last node points back [OK]

Hint: Circular means last node links back to earlier node [OK]

Common Mistakes:

• Thinking circular means sorted
• Confusing empty list with circular
• Assuming single node list is always circular

2. Which Rust code snippet correctly defines a singly linked list node struct?

easy

A. struct Node { value: i32, next: Option }

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

C. struct Node { value: i32, next: &Node }

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

Solution

1. Step 1: Understand Rust ownership for linked lists

   Rust requires using Option for next to allow nullable and heap allocation.

2. Step 2: Check each option

   struct Node { value: i32, next: Option } uses Option, which is correct. Others either lack Option or use references incorrectly.

3. Final Answer:

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

4. Quick Check:

   Use Option for next pointer in Rust [OK]

Hint: Use Option for next pointer in Rust [OK]

Common Mistakes:

• Using raw Node type for next causes infinite size error
• Using references without lifetimes
• Omitting Option for nullable next

3. Given the following Rust code to detect a circular linked list, what will be printed?

fn is_circular(head: &Option) -> bool {
    let mut slow = head.as_ref();
    let mut fast = head.as_ref();
    while let (Some(s), Some(f)) = (slow, fast) {
        fast = f.next.as_ref().and_then(|n| n.next.as_ref());
        slow = s.next.as_ref();
        if slow == fast {
            return true;
        }
    }
    false
}

// List: 1 -> 2 -> 3 -> 2 (loop back to node with value 2)
// Calling is_circular returns what?

medium

A. false

B. true

C. Compilation error

D. Infinite loop

Solution

1. Step 1: Understand Floyd's cycle detection

   Two pointers move at different speeds; if they meet, list is circular.

2. Step 2: Analyze given list and code

   List loops back from node 3 to node 2, so slow and fast pointers will meet, returning true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Floyd's algorithm detects loop = true [OK]

Hint: Floyd's cycle detection returns true if pointers meet [OK]

Common Mistakes:

• Confusing pointer movement speeds
• Not checking both pointers for null
• Assuming no loop when pointers meet

4. Identify the bug in this Rust function that tries to detect a circular linked list:

fn has_cycle(head: &Option) -> bool {
    let mut slow = head.as_ref();
    let mut fast = head.as_ref();
    while fast.is_some() && fast.unwrap().next.is_some() {
        slow = slow.unwrap().next.as_ref();
        fast = fast.unwrap().next.as_ref().unwrap().next.as_ref();
        if slow == fast {
            return true;
        }
    }
    false
}

medium

A. Function always returns false due to wrong loop condition

B. Pointers move at the same speed, so no cycle detection

C. Using unwrap() without checking can cause panic if null

D. Missing return type causes compilation error

Solution

1. Step 1: Check unwrap usage safety

   Using unwrap() without confirming Some can panic if null is encountered.

2. Step 2: Identify risk in loop

   fast.unwrap().next.as_ref().unwrap() assumes next exists, which may not be true at list end.

3. Final Answer:

   Using unwrap() without checking can cause panic if null -> Option C

4. Quick Check:

   Unsafe unwrap causes panic [OK]

Hint: Avoid unwrap without checking Option first [OK]

Common Mistakes:

• Ignoring unwrap panic risk
• Assuming pointers move same speed
• Not checking loop exit conditions

5. You have a linked list where the last node's next pointer is mistakenly set to itself, creating a single-node loop. How should you modify the Rust cycle detection function to correctly detect this case?

hard

A. Use a hash set to store visited nodes and detect repeats

B. Ignore cases where next points to itself, treat as non-circular

C. Only check if fast pointer reaches null to decide no cycle

D. Check if slow and fast pointers meet even if they start at the same node

Solution

1. Step 1: Understand single-node loop problem

   If a node points to itself, Floyd's algorithm may detect cycle, but edge cases can be tricky.

2. Step 2: Use hash set for robust detection

   Storing visited nodes in a hash set helps detect any repeated node, including self-loop cases.

3. Final Answer:

   Use a hash set to store visited nodes and detect repeats -> Option A

4. Quick Check:

   Hash set detects all cycles including self-loops [OK]

Hint: Use hash set to catch all loops including self-pointing nodes [OK]

Common Mistakes:

• Assuming Floyd's algorithm covers all edge cases
• Ignoring self-loop as cycle
• Not handling single-node circular lists