Detect if a Linked List is Circular in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the linked list has no next node (end).

DSA Rust

if current.next.is_[1]() {
    return false;
}

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

Aempty

Bnone

Csome

Attempts:

3 left

💡 Hint

Common Mistakes

Using is_some() instead of is_none() causes wrong logic.

Using a method that does not exist on Option.

The is_none() method checks if the Option is None, meaning no next node exists, so the list is not circular.

2fill in blank

medium

Complete the code to move the slow pointer one step forward.

DSA Rust

slow = slow.next.as_ref().map(|node| &**[1]).unwrap();

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

Afast

Bslow

Ccurrent

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Using the variable slow inside the closure instead of the argument.

Using fast or current which are unrelated here.

The closure receives node as the reference to the next node, so we dereference it to get the node itself.

3fill in blank

hard

Fix the error in the condition to detect if slow and fast pointers meet.

DSA Rust

if std::ptr::eq(slow, [1]) {
    return true;
}

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

Acurrent

Bnode

Cfast

Dslow

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing slow with itself always returns true incorrectly.

Using unrelated variables like current or node.

We compare the slow pointer with the fast pointer to detect a cycle.

4fill in blank

hard

Fill both blanks to advance the fast pointer two steps safely.

DSA Rust

fast = fast.next.as_ref().and_then(|node| node.next.as_ref().map(|[1]| &**[2])).unwrap_or(fast);

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

Anode

Bfast

Cnext_node

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both closures causes confusion.

Using unrelated variable names like fast or current inside closures.

The first closure argument is named node, and the second closure argument is named next_node to access the second next node.

5fill in blank

hard

Fill all three blanks to complete the function that detects if a linked list is circular.

DSA Rust

pub fn is_circular(head: Option<&Box<Node>>) -> bool {
    let mut slow = head.unwrap();
    let mut fast = head.unwrap();
    while fast.next.is_some() && fast.next.as_ref().unwrap().next.is_some() {
        slow = slow.next.as_ref().map(|[1]| &**[2]).unwrap();
        fast = fast.next.as_ref().and_then(|node| node.next.as_ref().map(|[3]| &**[3])).unwrap();
        if std::ptr::eq(slow, fast) {
            return true;
        }
    }
    false
}

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

Anode

Bslow

Cfast

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Using different argument names inconsistently.

Not advancing fast pointer correctly by two steps.

The closures use node as the argument name to access the next nodes. The fast pointer is advanced by two steps and compared with slow.

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