Traversal of Circular Linked List in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a circular linked list?

A circular linked list is a linked list where the last node points back to the first node, forming a circle. This means there is no null at the end.

beginner

How do you know when to stop traversing a circular linked list?

You stop when you reach the starting node again after moving forward. This prevents infinite loops.

intermediate

Why is traversal of a circular linked list different from a singly linked list?

Because a circular linked list has no null end, you must track the start node to know when to stop. In a singly linked list, you stop at null.

intermediate

Show the Rust code snippet to traverse a circular linked list safely.

Use a loop that visits nodes and stops when the current node is the start node again. Example: let mut current = start; loop { process current; current = current.next; if current == start { break; } }

beginner

What is a common mistake when traversing a circular linked list?

A common mistake is forgetting to check if you returned to the start node, causing an infinite loop.

What condition stops traversal in a circular linked list?

A After visiting 10 nodes

B When the current node's next is null

C When the current node is the start node again

D When the current node's value is zero

Traversal stops when the current node loops back to the start node, since there is no null in a circular linked list.

In Rust, how do you safely traverse a circular linked list?

A Traverse until current node is null

B Use a loop and break when current node equals start node

C Use recursion without base case

D Traverse only once without checking nodes

You use a loop and stop when you reach the start node again to avoid infinite loops.

What happens if you forget to check for the start node during traversal?

A Infinite loop occurs

B Traversal stops early

C Program crashes immediately

D Traversal skips nodes

Without checking, the loop never ends because the list is circular.

Which of these is true about circular linked lists?

A Last node points to the first node

B Last node points to null

C First node points to null

D Nodes are arranged in a tree

In circular linked lists, the last node points back to the first node.

Why might circular linked lists be useful?

A They use less memory than arrays

B They always sort elements automatically

C They are easier to implement than singly linked lists

D They allow continuous cycling through elements

Circular linked lists let you cycle through elements repeatedly without stopping.

Explain how to traverse a circular linked list and how to avoid infinite loops.

Think about what makes circular linked lists different from normal linked lists.

Write a simple Rust code outline to traverse a circular linked list and print each node's value.

Use a loop and compare current node with start node.

Practice

1. What is the main difference between a circular linked list and a regular singly linked list?

easy

A. In a circular linked list, nodes have two pointers instead of one.

B. In a circular linked list, the last node points back to the first node.

C. In a circular linked list, nodes are stored in an array.

D. In a circular linked list, nodes cannot be traversed.

Solution

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

   A circular linked list connects the last node back to the first node, forming a circle.

2. Step 2: Compare with a regular singly linked list

   A regular singly linked list ends with a null pointer, not looping back.

3. Final Answer:

   In a circular linked list, the last node points back to the first node. -> Option B

4. Quick Check:

   Last node points to first node = A [OK]

Hint: Remember: circular means last node links back to start [OK]

Common Mistakes:

• Thinking circular lists have two pointers per node
• Confusing circular linked list with doubly linked list
• Assuming nodes are stored in arrays

2. Which Rust code snippet correctly starts traversal of a circular linked list from the head node?

easy

A. let mut current = head;
loop {
println!("{}", current.data);
current = current.next.unwrap();
if current == head { break; }
}

B. let mut current = head;
while current != None {
println!("{}", current.data);
current = current.next;
}

C. for node in head {
println!("{}", node.data);
}

D. let mut current = head;
loop {
println!("{}", current.data);
current = current.next.unwrap();
}

Solution

1. Step 1: Identify correct loop structure for circular traversal

   let mut current = head; loop { println!("{}", current.data); current = current.next.unwrap(); if current == head { break; } } uses a loop that breaks when current returns to head, correctly handling circular traversal.

2. Step 2: Check other options for errors

   let mut current = head; while current != None { println!("{}", current.data); current = current.next; } uses while with None check, which is incorrect for circular lists. for node in head { println!("{}", node.data); } uses for loop on head, invalid syntax. let mut current = head; loop { println!("{}", current.data); current = current.next.unwrap(); } loops infinitely without break.

3. Final Answer:

   let mut current = head; loop { println!("{}", current.data); current = current.next.unwrap(); if current == head { break; } } -> Option A

4. Quick Check:

   Loop until current == head = A [OK]

Hint: Use loop with break when current equals head [OK]

Common Mistakes:

• Using while with None check in circular list
• Forgetting to break loop on returning to head
• Using invalid for loop syntax on linked list

3. What will be the output of the following Rust code that traverses a circular linked list with nodes containing values 10, 20, 30?

let mut current = head;
loop {
    print!("{} ", current.data);
    current = current.next.unwrap();
    if current == head { break; }
}

medium

A. Runtime error due to infinite loop

B. 10 20 30 10

C. 20 30 10

D. 10 20 30

Solution

1. Step 1: Understand traversal logic

   The loop prints current.data, moves to next, and breaks when current equals head again.

2. Step 2: Trace the output

   Starting at 10, prints 10, moves to 20, prints 20, moves to 30, prints 30, moves to 10 (head), breaks loop.

3. Final Answer:

   10 20 30 -> Option D

4. Quick Check:

   Print nodes once until back to head = D [OK]

Hint: Stop printing when current node equals head again [OK]

Common Mistakes:

• Printing extra head node again after loop
• Assuming infinite loop without break
• Starting traversal from wrong node

4. Identify the bug in this Rust code snippet for traversing a circular linked list:

let mut current = head;
while current != head {
    println!("{}", current.data);
    current = current.next.unwrap();
}

medium

A. The code causes infinite loop because no break condition.

B. The unwrap() causes panic if next is None.

C. The loop never executes because current == head initially.

D. The println! macro is used incorrectly.

Solution

1. Step 1: Analyze loop condition

   The while loop condition is current != head, but current starts as head, so condition is false immediately.

2. Step 2: Understand effect on execution

   Loop body never runs, so no traversal happens.

3. Final Answer:

   The loop never executes because current == head initially. -> Option C

4. Quick Check:

   Loop condition false at start = B [OK]

Hint: Check initial loop condition matches traversal logic [OK]

Common Mistakes:

• Using while current != head instead of loop with break
• Ignoring that current starts at head
• Assuming unwrap() always safe without checking

5. You want to sum all values in a circular linked list of integers using Rust. Which approach correctly avoids infinite loops and sums all nodes exactly once?

hard

A. Use a loop starting at head, add current.data, move to next, break when current == head again.

B. Use a while loop checking current != None, add current.data, move to next.

C. Use recursion without base case to traverse nodes and sum values.

D. Use a for loop over head assuming it implements Iterator.

Solution

1. Step 1: Understand circular list traversal for summation

   We must visit each node once and stop when we return to head to avoid infinite loops.

2. Step 2: Evaluate options

   Use a loop starting at head, add current.data, move to next, break when current == head again. uses loop with break on current == head, correctly summing once. Use a while loop checking current != None, add current.data, move to next. uses None check which is invalid for circular lists. Use recursion without base case to traverse nodes and sum values. causes infinite recursion. Use a for loop over head assuming it implements Iterator. assumes Iterator trait which is not standard for linked lists.

3. Final Answer:

   Use a loop starting at head, add current.data, move to next, break when current == head again. -> Option A

4. Quick Check:

   Loop with break on head = C [OK]

Hint: Break loop when current node equals head to avoid repeats [OK]

Common Mistakes:

• Using None checks in circular lists
• Recursing without base case causing stack overflow
• Assuming linked list implements Iterator trait