Traversal of Circular Linked List in DSA Java - 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. 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 through the list, because the list loops back to the start.

intermediate

Why can't you use 'null' to stop traversal in a Circular Linked List?

Because the last node points back to the first node, there is no null to indicate the end. You must track the start node to stop.

beginner

Show the basic steps to traverse a Circular Linked List.

1. Start at the head node.
2. Visit the current node.
3. Move to the next node.
4. Repeat until you reach the head node again.

beginner

What is the output of traversing a Circular Linked List with nodes 1 -> 2 -> 3 -> back to 1?

The output is: 1 -> 2 -> 3 -> (then stops because it reached 1 again).

In a Circular Linked List, what does the last node point to?

A Null

B A random node

C The first node

D Itself

The last node points back to the first node, creating a circle.

When traversing a Circular Linked List, how do you know when to stop?

A After visiting all nodes once and reaching the start node again

B When you reach a null pointer

C After visiting two nodes

D When the next node is itself

Traversal stops when you reach the starting node again after visiting all nodes.

Which of these is NOT true about Circular Linked Lists?

A They always have a head node

B The last node points to the first node

C They have no null at the end

D They can be traversed infinitely without a stop condition

Circular linked lists can be empty (head == null), so they do not always have a head node.

What is the first step in traversing a Circular Linked List?

A Find the last node

B Start at the head node

C Check for null

D Reverse the list

Traversal always starts at the head node.

If a Circular Linked List has nodes 5 -> 10 -> 15 -> back to 5, what is the correct traversal output?

A 10 -> 15 -> 5

B 5 -> 10 -> 15 -> 5 -> 10

C 5 -> 10 -> 15 -> null

D 5 -> 10 -> 15

Traversal prints each node once until it reaches the start node again, so output is 5 -> 10 -> 15.

Explain how to traverse a Circular Linked List and how to know when to stop.

Think about the circle shape and how to avoid infinite loops.

Describe the difference between traversing a normal linked list and a circular linked list.

Focus on what marks the end of the list in each case.

Practice

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

easy

A. In a singly linked list, traversal stops when the starting node is reached again.

B. In a circular linked list, traversal stops when the last node's next is null.

C. Traversal is the same for both lists.

D. In a circular linked list, traversal stops when the starting node is reached again.

Solution

1. Step 1: Understand traversal stopping condition for singly linked list

   Traversal stops when the next pointer is null, indicating the end of the list.

2. Step 2: Understand traversal stopping condition for circular linked list

   Traversal stops when the traversal reaches the starting node again, since the list loops back.

3. Final Answer:

   In a circular linked list, traversal stops when the starting node is reached again. -> Option D

4. Quick Check:

   Stopping condition differs: null vs start node [OK]

Hint: Remember circular lists loop back to start node [OK]

Common Mistakes:

• Stopping traversal at null in circular list
• Confusing stopping conditions between list types
• Assuming circular list ends with null pointer

2. Which of the following Java code snippets correctly starts traversal of a circular linked list from the head node?

easy

A. Node current = head; do { System.out.println(current.data); current = current.next; } while(current != head);

B. Node current = head; while(current != null) { System.out.println(current.data); current = current.next; }

C. Node current = head.next; while(current != head) { System.out.println(current.data); current = current.next; }

D. Node current = null; do { System.out.println(current.data); current = current.next; } while(current != head);

Solution

1. Step 1: Check initialization and loop condition

   Node current = head; do { System.out.println(current.data); current = current.next; } while(current != head); initializes current to head and uses a do-while loop to print and move until current equals head again, which is correct for circular lists.

2. Step 2: Analyze other options

   Node current = head; while(current != null) { System.out.println(current.data); current = current.next; } uses while(current != null), which is incorrect for circular lists as next is never null. Node current = head.next; while(current != head) { System.out.println(current.data); current = current.next; } starts from head.next and misses printing head data. Node current = null; do { System.out.println(current.data); current = current.next; } while(current != head); initializes current to null, causing NullPointerException.

3. Final Answer:

   Node current = head; do { System.out.println(current.data); current = current.next; } while(current != head); -> Option A

4. Quick Check:

   Use do-while with current != head [OK]

Hint: Use do-while loop starting at head for circular lists [OK]

Common Mistakes:

• Using while(current != null) for circular lists
• Starting traversal from head.next and missing head node
• Initializing current to null causing errors

3. What will be the output of the following code snippet for a circular linked list with nodes containing data 10 -> 20 -> 30 -> back to 10?

Node head = new Node(10);
head.next = new Node(20);
head.next.next = new Node(30);
head.next.next.next = head;

Node current = head;
do {
    System.out.print(current.data + " ");
    current = current.next;
} while(current != head);

medium

A. 10 20 30 10

B. 10 20 30

C. 20 30 10

D. 10 20 30 30

Solution

1. Step 1: Understand the circular linked list structure

   The list nodes are 10 -> 20 -> 30, and 30 points back to 10, making it circular.

2. Step 2: Trace the do-while loop output

   Starting at head (10), print 10, move to 20, print 20, move to 30, print 30, move to 10 again. Loop stops when current == head, so 10 is not printed again.

3. Final Answer:

   10 20 30 -> Option B

4. Quick Check:

   Print nodes once until back to start [OK]

Hint: Do-while prints all nodes once before stopping [OK]

Common Mistakes:

• Printing the start node twice
• Missing the first node in output
• Printing extra nodes beyond the loop

4. Identify the error in the following code snippet for traversing a circular linked list:

Node current = head;
while(current != head) {
    System.out.print(current.data + " ");
    current = current.next;
}

medium

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

B. The loop causes infinite printing of nodes.

C. The code throws NullPointerException.

D. The loop skips the last node.

Solution

1. Step 1: Analyze loop condition with initial current value

   current is initialized to head, so current != head is false at start, loop body never runs.

2. Step 2: Understand correct loop for circular list traversal

   Use do-while loop to ensure at least one iteration, since current starts at head.

3. Final Answer:

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

4. Quick Check:

   while(current != head) false at start [OK]

Hint: Use do-while to run loop at least once [OK]

Common Mistakes:

• Using while instead of do-while for circular lists
• Not printing the first node
• Assuming loop runs when condition false initially

5. Given a circular linked list where each node contains an integer, write a method to find the sum of all node values. Which approach below correctly handles the traversal without infinite loop?

hard

A. Initialize sum=0; Node current=head.next; while(current != head) { sum += current.data; current = current.next; } return sum;

B. Initialize sum=0; Node current=head; while(current != null) { sum += current.data; current = current.next; } return sum;

C. Initialize sum=0; Node current=head; do { sum += current.data; current = current.next; } while(current != head); return sum;

D. Initialize sum=0; Node current=null; do { sum += current.data; current = current.next; } while(current != head); return sum;

Solution

1. Step 1: Understand traversal stopping condition for circular list

   Traversal must stop when current returns to head to avoid infinite loop.

2. Step 2: Analyze each option's traversal logic

   Initialize sum=0; Node current=head; do { sum += current.data; current = current.next; } while(current != head); return sum; uses do-while starting at head, summing all nodes once. Initialize sum=0; Node current=head; while(current != null) { sum += current.data; current = current.next; } return sum; uses while(current != null), which never happens in circular list. Initialize sum=0; Node current=head.next; while(current != head) { sum += current.data; current = current.next; } return sum; starts at head.next, missing head's data. Initialize sum=0; Node current=null; do { sum += current.data; current = current.next; } while(current != head); return sum; initializes current to null causing error.

3. Final Answer:

   Initialize sum=0; Node current=head; do { sum += current.data; current = current.next; } while(current != head); return sum; -> Option C

4. Quick Check:

   Use do-while with current != head for full traversal [OK]

Hint: Sum with do-while loop until current equals head [OK]

Common Mistakes:

• Using while(current != null) causing infinite loop
• Skipping head node in sum calculation
• Initializing current to null causing errors