Traversal of Circular Linked List in DSA Java

Mental Model

A circular linked list connects the last node back to the first, so traversal loops until we return to the start.

Analogy: Imagine people sitting in a circle passing a ball; you keep passing until the ball comes back to the first person.

1 -> 2 -> 3 ->
↑           ↓
←←←←←←←←←←←

Dry Run Walkthrough

Input: list: 1 -> 2 -> 3 (circular), traverse and print all nodes

Goal: Visit each node exactly once and print its value

Step 1: start at head node with value 1

1 [curr↑] -> 2 -> 3 -> back to 1

Why: We begin traversal from the first node

Step 2: print current node value 1 and move curr to next node

1 -> 2 [curr↑] -> 3 -> back to 1

Why: Print current and move forward to visit next node

Step 3: print current node value 2 and move curr to next node

1 -> 2 -> 3 [curr↑] -> back to 1

Why: Continue visiting nodes in order

Step 4: print current node value 3 and move curr to next node

1 -> 2 -> 3 -> 1 [curr↑]

Why: After last node, curr points back to head

Step 5: stop traversal because curr is back at head

1 -> 2 -> 3 -> back to 1 (stop)

Why: We have visited all nodes once; stop to avoid infinite loop

Result:

1 -> 2 -> 3 -> back to 1 (traversal complete)

Annotated Code

DSA Java

class Node {
    int data;
    Node next;
    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

public class CircularLinkedListTraversal {
    Node head;

    // Method to traverse and print circular linked list
    void traverse() {
        if (head == null) return; // empty list check
        Node curr = head;
        do {
            System.out.print(curr.data + " -> "); // print current node
            curr = curr.next; // move to next node
        } while (curr != head); // stop when back to head
        System.out.print("head\n"); // end of traversal
    }

    public static void main(String[] args) {
        CircularLinkedListTraversal list = new CircularLinkedListTraversal();
        // create nodes
        list.head = new Node(1);
        Node second = new Node(2);
        Node third = new Node(3);
        // link nodes in circular way
        list.head.next = second;
        second.next = third;
        third.next = list.head;

        list.traverse();
    }
}

if (head == null) return; // empty list check

stop if list is empty to avoid errors

Node curr = head;

start traversal from head node

do {

begin loop to visit nodes at least once

System.out.print(curr.data + " -> ");

print current node's data

curr = curr.next;

advance curr to next node

} while (curr != head);

continue until we return to head node

System.out.print("head\n");

print head to mark end of traversal

Output

1 -> 2 -> 3 -> head

Complexity Analysis

Time: O(n) because we visit each of the n nodes exactly once during traversal

Space: O(1) because we use only a few pointers regardless of list size

vs Alternative: Compared to linear linked list traversal, circular traversal requires a stopping condition to avoid infinite loops, but time complexity remains O(n)

Edge Cases

empty list (head is null)

traversal immediately returns without printing

DSA Java

if (head == null) return; // empty list check

single node circular list (node points to itself)

prints the single node once and stops

DSA Java

do { ... } while (curr != head); // stops after one full loop

When to Use This Pattern

When you see a linked list where the last node points back to the first, use circular linked list traversal with a do-while loop to visit all nodes exactly once without infinite looping.

Common Mistakes

Mistake: Using a while loop with condition curr != null, which never ends in circular lists
Fix: Use a do-while loop with condition curr != head to stop after one full cycle

Mistake: Not checking for empty list before traversal causing null pointer errors
Fix: Add a check if head is null before starting traversal

Summary

It visits each node in a circular linked list exactly once by looping until it returns to the start.

Use it when you need to process or print all elements in a circular linked list safely.

The key insight is to stop traversal when you reach the starting node again to avoid infinite loops.

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