Traversal of Circular Linked List in DSA Java - Build from Scratch

Traversal of Circular Linked List

📖 Scenario: Imagine you have a group of friends sitting around a round table. You want to greet each friend one by one, starting from a specific person and going around the table until you come back to the start. This is like traversing a circular linked list.

🎯 Goal: You will create a circular linked list with 4 nodes, then write code to traverse and print all the node values starting from the head node, stopping after one full circle.

📋 What You'll Learn

Create a circular linked list with nodes containing values 10, 20, 30, and 40

Set the last node's next pointer to the head node to form a circle

Write a traversal loop that prints each node's value exactly once

Print the node values separated by spaces on a single line

💡 Why This Matters

🌍 Real World

Circular linked lists are used in real life for applications like music playlists that loop, or managing players in a board game where turns cycle continuously.

💼 Career

Understanding circular linked lists helps in software development roles that involve designing efficient data structures for cyclic processes or real-time systems.

1

Create Circular Linked List Nodes

Create a class called Node with an integer data and a Node reference called next. Then create four Node objects named node1, node2, node3, and node4 with data values 10, 20, 30, and 40 respectively.

DSA Java

// Define Node class and create four nodes with data 10, 20, 30, 40
// Your code here

Hint

Define the Node class first, then create each node with the given values.

2

Link Nodes to Form Circular List

Link the nodes so that node1.next points to node2, node2.next points to node3, node3.next points to node4, and node4.next points back to node1 to complete the circle. Also, create a Node variable called head and assign it to node1.

DSA Java

class Node {
    int data;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node(10);
        Node node2 = new Node(20);
        Node node3 = new Node(30);
        Node node4 = new Node(40);

        // Link nodes to form a circular linked list
        // Your code here
    }
}

Hint

Set each node's next to the following node, and the last node's next back to the first node.

3

Traverse the Circular Linked List

Write a Node variable called current and set it to head. Use a do-while loop to print the data of each node followed by a space. Move current to current.next each time. Stop the loop when current becomes head again after the first iteration.

DSA Java

class Node {
    int data;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node(10);
        Node node2 = new Node(20);
        Node node3 = new Node(30);
        Node node4 = new Node(40);

        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node1;

        Node head = node1;

        // Traverse and print the circular linked list
        // Your code here
    }
}

Hint

Use a do-while loop to ensure the loop runs at least once and stops after one full circle.

4

Print the Traversal Output

Run the program to print the data values of the circular linked list nodes separated by spaces on one line.

DSA Java

class Node {
    int data;
    Node next;

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

public class Main {
    public static void main(String[] args) {
        Node node1 = new Node(10);
        Node node2 = new Node(20);
        Node node3 = new Node(30);
        Node node4 = new Node(40);

        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        node4.next = node1;

        Node head = node1;

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

        // No additional code needed
    }
}

Hint

Run the program and check that the output matches the expected sequence.

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