Create a Circular Singly Linked List in DSA C++ - Execution Trace

Concept Flow - Create a Circular Singly Linked List

Create new node with data

↓

Is list empty?

No→Traverse to last node (node.next != head)

↓

Set last node.next = new node

↓

Set head = new node

↓

Set new node.next = head

↓

Done

Create a new node, if list is empty set head to new node and point it to itself. Otherwise, find last node and link it to new node, then link new node back to head.

Execution Sample

DSA C++

Node* head = nullptr;
Node* newNode = new Node(10);
if (!head) {
  head = newNode;
  newNode->next = head;
}

Create a circular singly linked list with one node pointing to itself.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	None	newNode=nullptr->Node(10)	null
2	Check if list is empty (head == nullptr)	None	head=nullptr	null
3	List empty: set head = newNode	Node(10)	head=nullptr->Node(10)	Node(10) -> ?
4	Set newNode->next = head (self-loop)	Node(10)	newNode->next = head	Node(10) -> Node(10) (circular)
5	Done	Node(10)	head points to Node(10)	Node(10) -> Node(10) (circular)

💡 List created with one node pointing to itself, forming a circular singly linked list.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
head	nullptr	nullptr	Node(10)	Node(10)	Node(10)
newNode	nullptr	Node(10)	Node(10)	Node(10)	Node(10)
newNode->next	undefined	undefined	undefined	Node(10)	Node(10)

Key Moments - 3 Insights

Why does the new node point to itself when the list is empty?

Because in a circular singly linked list with one node, the node's next pointer must point back to itself to maintain the circular structure, as shown in execution_table step 4.

What happens if we forget to set newNode->next = head?

The list will not be circular; the new node's next pointer will be undefined or null, breaking the circular property. This is why step 4 is crucial.

Why do we check if head is nullptr before inserting?

Because if the list is empty (head == nullptr), we must initialize head and create the circular link differently than when adding nodes to a non-empty list, as shown in step 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of head after step 3?

A nullptr

B Node(10)

C undefined

D Node(20)

💡 Hint

Refer to execution_table row with Step 3 under Pointer Changes and Nodes in List columns.

At which step does the new node's next pointer get set to point to head?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check execution_table for the operation describing setting newNode->next = head.

If the list was not empty, which step would be missing from this execution?

A Step 3 (Set head = newNode)

B Step 1 (Create new node)

C Step 4 (Set newNode->next = head)

D Step 5 (Done)

💡 Hint

Look at the condition for empty list and what happens when list is not empty in concept_flow.

Concept Snapshot

Create a new node.
If list empty: head = new node, node.next = head (self-loop).
Else: traverse to last node, last.next = new node, new node.next = head.
This forms a circular singly linked list.
Always maintain circular link to head.

Full Transcript

To create a circular singly linked list, start by creating a new node with the given data. Check if the list is empty by seeing if head is nullptr. If empty, set head to the new node and point the new node's next to head itself, making a self-loop. If not empty, traverse the list to find the last node (the one whose next points to head), then set last node's next to the new node, and new node's next to head. This keeps the list circular. The execution table shows these steps with the nodes and pointer changes. The variable tracker shows how head and newNode pointers change. Key moments clarify why the new node points to itself when list is empty and why setting newNode->next is crucial. The visual quiz tests understanding of these steps.

Practice

1. What is a key characteristic of a circular singly linked list compared to a regular singly linked list?

easy

A. It uses an array internally to store nodes.

B. Each node has two pointers, one to the next and one to the previous node.

C. It cannot be traversed starting from the head node.

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

Solution

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

   In a circular singly linked list, the last node's next pointer points back to the first node, creating a loop.

2. Step 2: Compare with other linked lists

   Unlike doubly linked lists (which have two pointers) or arrays, circular singly linked lists have a single pointer per node and form a circle.

3. Final Answer:

   The last node points back to the first node, forming a circle. -> Option D

4. Quick Check:

   Circular list = last node points to first node [OK]

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

Common Mistakes:

• Thinking circular means two pointers per node
• Confusing with doubly linked list
• Assuming it uses arrays internally

2. Which of the following is the correct way to declare a node structure for a circular singly linked list in C++?

easy

A. struct Node { int data; int next; }; // Incorrect pointer type

B. struct Node { int data; Node* prev; Node* next; }; // Doubly linked list node

C. struct Node { int data; Node* next; }; // Correct declaration

D. struct Node { int data; Node next; }; // Incorrect pointer usage

Solution

1. Step 1: Identify the correct node structure for singly linked list

   A singly linked list node has data and a pointer to the next node. The pointer type must be Node*.

2. Step 2: Check each option

   struct Node { int data; Node* next; }; // Correct declaration correctly declares Node* next. struct Node { int data; Node* prev; Node* next; }; // Doubly linked list node has an extra prev pointer (doubly linked list). struct Node { int data; int next; }; // Incorrect pointer type uses int next which is wrong type. struct Node { int data; Node next; }; // Incorrect pointer usage uses Node next which is not a pointer.

3. Final Answer:

   struct Node { int data; Node* next; }; -> Option C

4. Quick Check:

   Single pointer to next node = correct node structure [OK]

Hint: Node must have pointer to next node only [OK]

Common Mistakes:

• Using non-pointer type for next
• Adding prev pointer for singly linked list
• Declaring next as object instead of pointer

3. What will be the output of the following code snippet that creates a circular singly linked list with 3 nodes and prints their data?

struct Node {
    int data;
    Node* next;
};

int main() {
    Node* head = new Node{1, nullptr};
    Node* second = new Node{2, nullptr};
    Node* third = new Node{3, nullptr};

    head->next = second;
    second->next = third;
    third->next = head; // circular link

    Node* temp = head;
    for (int i = 0; i < 5; i++) {
        std::cout << temp->data << " ";
        temp = temp->next;
    }
    return 0;
}

medium

A. 1 2 3 1 2

B. 1 2 3 3 3

C. 1 2 3 4 5

D. 1 2 3 1 1

Solution

1. Step 1: Understand the circular link setup

   The last node (third) points back to head, so the list loops: 1 -> 2 -> 3 -> 1 -> ...

2. Step 2: Trace the print loop for 5 iterations

   Starting at head (1), print 1, move to 2, print 2, move to 3, print 3, move to 1, print 1, move to 2, print 2.

3. Final Answer:

   1 2 3 1 2 -> Option A

4. Quick Check:

   Loop prints nodes in circle: 1 2 3 1 2 [OK]

Hint: Circular list repeats nodes after last one [OK]

Common Mistakes:

• Assuming list ends after 3 nodes
• Printing wrong data due to pointer errors
• Confusing circular with linear list output

4. Identify the error in the following code that attempts to create a circular singly linked list with one node:

struct Node {
    int data;
    Node* next;
};

int main() {
    Node* head = new Node{10, nullptr};
    head->next = nullptr; // Intended to make circular
    return 0;
}

medium

A. Memory leak due to missing delete statement.

B. head->next should point to head, not nullptr, to form a circular list.

C. Using nullptr for next is correct for circular list.

D. Node declaration is missing a constructor.

Solution

1. Step 1: Check how circular link is formed

   For a circular singly linked list with one node, the node's next pointer must point to itself.

2. Step 2: Identify the mistake in the code

   The code sets head->next = nullptr, which breaks the circular property.

3. Final Answer:

   head->next should point to head, not nullptr, to form a circular list. -> Option B

4. Quick Check:

   One node circular list: next points to itself [OK]

Hint: One node circular list points next to itself [OK]

Common Mistakes:

• Setting next to nullptr instead of self
• Confusing circular with linear list
• Ignoring memory management issues (not relevant here)

5. You want to insert a new node with value 4 at the end of an existing circular singly linked list with nodes 1 -> 2 -> 3 -> back to 1. Which of the following code snippets correctly performs this insertion?

hard

A. Node* newNode = new Node{4, head};
Node* temp = head;
while (temp->next != head) temp = temp->next;
temp->next = newNode;

B. Node* newNode = new Node{4, nullptr};
Node* temp = head;
while (temp->next != nullptr) temp = temp->next;
temp->next = newNode;
newNode->next = head;

C. Node* newNode = new Node{4, head};
head->next = newNode;

D. Node* newNode = new Node{4, nullptr};
newNode->next = head;
head = newNode;

Solution

1. Step 1: Understand insertion at end in circular singly linked list

   To insert at end, find last node (whose next points to head), then link new node to head and last node to new node.

2. Step 2: Analyze each option

   Node* newNode = new Node{4, head}; Node* temp = head; while (temp->next != head) temp = temp->next; temp->next = newNode; correctly creates new node pointing to head, finds last node by looping until temp->next == head, then sets last node's next to new node. Node* newNode = new Node{4, nullptr}; Node* temp = head; while (temp->next != nullptr) temp = temp->next; temp->next = newNode; newNode->next = head; incorrectly checks for nullptr which does not exist in circular list. Node* newNode = new Node{4, head}; head->next = newNode; incorrectly sets head->next to new node, breaking list. Node* newNode = new Node{4, nullptr}; newNode->next = head; head = newNode; changes head pointer incorrectly and does not update last node's next.

3. Final Answer:

   Node* newNode = new Node{4, head}; Node* temp = head; while (temp->next != head) temp = temp->next; temp->next = newNode; -> Option A

4. Quick Check:

   Find last node by next == head, link new node to head, last node to new node [OK]

Hint: Loop until next points to head to find last node [OK]

Common Mistakes:

• Checking for nullptr in circular list
• Not updating last node's next pointer
• Incorrectly changing head pointer