Circular linked list in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a circular linked list?

A circular linked list is a type of linked list where the last node points back to the first node, forming a circle. This means you can traverse the list starting from any node and eventually come back to it.

beginner

How does a circular linked list differ from a singly linked list?

In a singly linked list, the last node points to null (no next node), ending the list. In a circular linked list, the last node points back to the first node, creating a loop.

intermediate

What is one advantage of using a circular linked list?

One advantage is that you can easily loop through the list continuously without needing to restart from the head. This is useful for applications like round-robin scheduling or buffering.

intermediate

How do you detect the end of traversal in a circular linked list?

Since there is no null to mark the end, you detect the end by checking if you have returned to the starting node during traversal.

intermediate

Can a circular linked list be singly or doubly linked?

Yes, a circular linked list can be singly linked (each node points to the next) or doubly linked (each node points to both next and previous nodes), but in both cases the last node connects back to the first.

In a circular linked list, what does the last node point to?

A The middle node

B Null

C Itself

D The first node

The last node in a circular linked list points back to the first node, creating a loop.

Which of these is a common use case for circular linked lists?

A Binary search

B Sorting numbers

C Round-robin scheduling

D Storing hierarchical data

Circular linked lists are useful for round-robin scheduling because they allow continuous cycling through elements.

How do you know when you have traversed all nodes in a circular linked list?

A When you return to the starting node

B When the list is empty

C After a fixed number of steps

D When you reach a null pointer

Traversal ends when you come back to the node where you started, since there is no null pointer.

Can a circular linked list have only one node?

A Yes, and it points to itself

B No, it must have at least two nodes

C Yes, but it points to null

D No, it must be empty

A circular linked list with one node points that node's next pointer back to itself.

What is a key difference between singly and doubly circular linked lists?

A Singly circular linked lists have no next pointers

B Doubly circular linked lists have pointers to both next and previous nodes

C Doubly circular linked lists do not form a loop

D Singly circular linked lists have two heads

Doubly circular linked lists have two pointers per node: one to the next node and one to the previous node.

Explain what a circular linked list is and how it works.

Think about how the list forms a circle instead of ending.

Describe one practical use of a circular linked list and why it is suitable.

Consider situations where you need to repeat tasks evenly.

Practice

1. What is the main characteristic of a circular linked list?

easy

A. The last node points back to the first node, forming a loop.

B. It has a fixed size and cannot grow.

C. Nodes are connected in a straight line with no loops.

D. It stores data in a sorted order automatically.

Solution

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

   A circular linked list connects its last node back to the first node, creating a loop.

2. Step 2: Compare with other list types

   Unlike linear linked lists, circular linked lists do not end with a null pointer but loop back to the start.

3. Final Answer:

   The last node points back to the first node, forming a loop. -> Option A

4. Quick Check:

   Circular linked list = loop connection [OK]

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

Common Mistakes:

• Thinking it has a fixed size
• Confusing with linear linked list
• Assuming automatic sorting

2. Which of the following is the correct way to check if a circular linked list is empty?

easy

A. Check if the head's next pointer is null.

B. Check if the last node points to null.

C. Check if the list has only one node.

D. Check if the head pointer is null.

Solution

1. Step 1: Identify how an empty circular linked list is represented

   An empty circular linked list has no nodes, so the head pointer is null.

2. Step 2: Understand pointer connections in circular lists

   In circular lists, the last node points to the head, never null, so checking last node's pointer is incorrect.

3. Final Answer:

   Check if the head pointer is null. -> Option D

4. Quick Check:

   Empty list = head is null [OK]

Hint: Empty means head pointer is null, not next pointers [OK]

Common Mistakes:

• Checking last node's pointer for null
• Assuming single node means empty
• Checking head's next pointer instead of head

3. Consider a circular linked list with nodes containing values 10, 20, 30 in that order. If you start at the node with value 10 and move forward 4 times, which node value will you be at?

medium

A. 20

B. 10

C. 30

D. None, it will cause an error

Solution

1. Step 1: Understand the circular traversal

   The list nodes are 10 -> 20 -> 30 -> back to 10, forming a loop.

2. Step 2: Move forward 4 times starting at 10

   Moving 1 step: 20, 2 steps: 30, 3 steps: 10, 4 steps: 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   4 steps in 3-node loop lands on 20 [OK]

Hint: Use modulo steps by list size to find position [OK]

Common Mistakes:

• Stopping at 30 instead of continuing loop
• Assuming error on extra steps
• Counting starting node as step zero incorrectly

4. You have a circular linked list but the last node's next pointer is set to null instead of the head. What problem will this cause?

medium

A. The list will become empty.

B. The list will automatically fix the pointer to head.

C. The list will behave like a linear linked list and lose circular properties.

D. The list will cause a syntax error.

Solution

1. Step 1: Identify the pointer issue in circular linked list

   The last node must point to the head to maintain circularity; pointing to null breaks the loop.

2. Step 2: Understand the effect of last node pointing to null

   With last node pointing to null, the list acts like a linear linked list ending at that node.

3. Final Answer:

   The list will behave like a linear linked list and lose circular properties. -> Option C

4. Quick Check:

   Last node null breaks circular loop [OK]

Hint: Last node must point to head, not null [OK]

Common Mistakes:

• Assuming automatic pointer fix
• Thinking list becomes empty
• Confusing runtime errors with pointer issues

5. You want to implement a music playlist that repeats songs endlessly using a circular linked list. Which approach ensures the playlist cycles correctly without stopping?

hard

A. Set the last song node's next pointer to null to mark the end.

B. Link the last song node's next pointer back to the first song node.

C. Use a doubly linked list with no circular links.

D. Store all songs in an array instead of a linked list.

Solution

1. Step 1: Understand the requirement for endless cycling

   To repeat songs endlessly, the playlist must loop back to the start after the last song.

2. Step 2: Apply circular linked list property

   Linking the last song's next pointer to the first song creates a circular linked list that cycles indefinitely.

3. Step 3: Evaluate other options

   Setting next to null stops the list; doubly linked or arrays don't guarantee endless cycling without extra logic.

4. Final Answer:

   Link the last song node's next pointer back to the first song node. -> Option B

5. Quick Check:

   Last node points to first for endless cycle [OK]

Hint: Last node points to first for infinite loop [OK]

Common Mistakes:

• Setting last node next to null
• Using non-circular lists for cycling
• Confusing data structure choice with cycling logic