Insert at Beginning of Circular Linked List in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after inserting a node at the beginning

What is the printed state of the circular linked list after inserting 10 at the beginning of the list containing 20 -> 30 -> 40?

DSA C

/* Circular linked list node structure */
#include <stdio.h>
#include <stdlib.h>

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

/* Function to insert at beginning */
Node* insertAtBeginning(Node* head, int data) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = data;
    if (head == NULL) {
        newNode->next = newNode;
        return newNode;
    }
    Node* temp = head;
    while (temp->next != head) {
        temp = temp->next;
    }
    temp->next = newNode;
    newNode->next = head;
    return newNode;
}

/* Function to print circular linked list */
void printList(Node* head) {
    if (head == NULL) return;
    Node* temp = head;
    do {
        printf("%d -> ", temp->data);
        temp = temp->next;
    } while (temp != head);
    printf("null\n");
}

int main() {
    Node* head = (Node*)malloc(sizeof(Node));
    Node* second = (Node*)malloc(sizeof(Node));
    Node* third = (Node*)malloc(sizeof(Node));

    head->data = 20;
    second->data = 30;
    third->data = 40;

    head->next = second;
    second->next = third;
    third->next = head;

    head = insertAtBeginning(head, 10);
    printList(head);
    return 0;
}

A 10 -> 30 -> 40 -> 20 -> null

B 20 -> 30 -> 40 -> 10 -> null

C 10 -> 20 -> 30 -> 40 -> null

D 20 -> 10 -> 30 -> 40 -> null

💡 Hint

Remember that in a circular linked list, the last node points back to the head. Inserting at the beginning means the new node becomes the new head.

The function inserts a new node before the current head and updates the last node's next pointer to the new node. Thus, the new node with data 10 becomes the head, followed by 20, 30, and 40.

🧠 Conceptual

intermediate

Understanding pointer updates in circular linked list insertion

Which pointer update is essential to maintain the circular nature of the list when inserting a new node at the beginning?

A Updating the new node's next pointer to point to the old head and the last node's next pointer to the new node

B Updating the new node's next pointer to NULL and the last node's next pointer to the new node

C Updating the last node's next pointer to NULL and the new node's next pointer to the old head

D Updating the old head's next pointer to the new node and the new node's next pointer to NULL

💡 Hint

Think about how the last node should point after insertion to keep the list circular.

To keep the list circular, the last node must point to the new head (new node), and the new node must point to the old head. This maintains the loop.

🔧 Debug

advanced

Identify the bug in this insertion code

What error will occur when running this code to insert at the beginning of a circular linked list?

DSA C

Node* insertAtBeginning(Node* head, int data) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = data;
    newNode->next = head;
    if (head == NULL) {
        head = newNode;
        return head;
    }
    Node* temp = head;
    while (temp->next != head) {
        temp = temp->next;
    }
    temp->next = newNode;
    return newNode;
}

A Infinite loop or segmentation fault due to incorrect loop condition

B No error, code runs correctly

C Memory leak due to missing free() calls

D Compilation error due to missing semicolon

💡 Hint

Check the loop condition for traversing the circular linked list.

The loop condition uses temp->next != NULL, but in a circular linked list, the last node points to head, not NULL. This causes an infinite loop or crash.

❓ Predict Output

advanced

Output after multiple insertions at beginning

What is the printed state of the circular linked list after inserting 50, then 40, then 30 at the beginning of an empty list?

DSA C

/* Insert multiple nodes at beginning */
#include <stdio.h>
#include <stdlib.h>

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

Node* insertAtBeginning(Node* head, int data) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = data;
    if (head == NULL) {
        newNode->next = newNode;
        return newNode;
    }
    Node* temp = head;
    while (temp->next != head) {
        temp = temp->next;
    }
    temp->next = newNode;
    newNode->next = head;
    return newNode;
}

void printList(Node* head) {
    if (head == NULL) return;
    Node* temp = head;
    do {
        printf("%d -> ", temp->data);
        temp = temp->next;
    } while (temp != head);
    printf("null\n");
}

int main() {
    Node* head = NULL;
    head = insertAtBeginning(head, 50);
    head = insertAtBeginning(head, 40);
    head = insertAtBeginning(head, 30);
    printList(head);
    return 0;
}

A 50 -> 40 -> 30 -> null

B 30 -> 40 -> 50 -> null

C 40 -> 30 -> 50 -> null

D 30 -> 50 -> 40 -> null

💡 Hint

Each insertion at beginning makes the new node the head.

Inserting 50 first creates list: 50 -> null. Then inserting 40 at beginning: 40 -> 50 -> null. Then inserting 30 at beginning: 30 -> 40 -> 50 -> null.

🧠 Conceptual

expert

Time complexity of insertion at beginning in circular linked list

What is the time complexity of inserting a node at the beginning of a circular linked list when you only have a pointer to the head node?

A O(n^2) because of nested traversal

B O(1) because you can insert immediately at the head without traversal

C O(log n) because of binary search on nodes

D O(n) because you must traverse to the last node to update its next pointer

💡 Hint

Think about what pointer updates are needed and if you can access the last node directly.

To insert at beginning, you must update the last node's next pointer to the new node. Without a direct pointer to the last node, you must traverse the list, which takes O(n) time.

Practice

1. What happens when you insert a new node at the beginning of a circular linked list?

easy

A. The new node is added at the end of the list, breaking the circular link.

B. The new node becomes the head and points to the old head, maintaining the circular link.

C. The new node replaces the last node in the list.

D. The list becomes a linear linked list instead of circular.

Solution

1. Step 1: Understand the circular linked list structure

   A circular linked list has its last node pointing back to the head node, forming a circle.

2. Step 2: Insert new node at beginning

   The new node is linked to the current head, and the last node's next pointer is updated to this new node, making it the new head.

3. Final Answer:

   The new node becomes the head and points to the old head, maintaining the circular link. -> Option B

4. Quick Check:

   New head points to old head, circle intact [OK]

Hint: New head points to old head; last node points to new head [OK]

Common Mistakes:

• Forgetting to update last node's next pointer
• Breaking the circular link by not connecting last node
• Replacing the last node instead of adding new head

2. Which of the following is the correct syntax to create a new node and insert it at the beginning of a circular linked list in C?

easy

A. head = new_node; new_node->next = head; last->next = head;

B. head = new_node; new_node->next = last; last = head;

C. last->next = new_node; new_node->next = head; head = new_node;

D. new_node->next = NULL; head = new_node; last->next = NULL;

Solution

1. Step 1: Link new node to current head

   Set new_node->next to head so new node points to old first node.

2. Step 2: Update last node's next pointer and head

   Set last->next to new_node to maintain circular link, then update head to new_node.

3. Final Answer:

   last->next = new_node; new_node->next = head; head = new_node; -> Option C

4. Quick Check:

   New node links head and last points to new node [OK]

Hint: Link new node between last and head, then update head [OK]

Common Mistakes:

• Setting new_node->next to NULL breaks circular link
• Updating head before linking last node
• Confusing order of pointer assignments

3. What will be the output of the following code snippet after inserting 10 at the beginning of the circular linked list?

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

// Existing list: 20 -> 30 -> 40 -> back to 20
struct Node* head = node20;
struct Node* last = node40;

// Insert 10 at beginning
struct Node* new_node = malloc(sizeof(struct Node));
new_node->data = 10;
new_node->next = head;
last->next = new_node;
head = new_node;

// Print list starting from head (print 4 nodes)

medium

A. 10 -> 20 -> 30 -> 40 -> back to 10

B. 20 -> 30 -> 40 -> 10 -> back to 20

C. 10 -> 30 -> 40 -> 20 -> back to 10

D. 30 -> 40 -> 20 -> 10 -> back to 30

Solution

1. Step 1: Insert new node at beginning

   New node with data 10 points to old head (20), last node (40) points to new node, head updated to new node.

2. Step 2: Print nodes starting from new head

   Printing 4 nodes from head: 10, 20, 30, 40, then back to 10.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> back to 10 -> Option A

4. Quick Check:

   New head 10 followed by old nodes in order [OK]

Hint: New node becomes head; last points to new head [OK]

Common Mistakes:

• Not updating last->next to new node
• Printing old head as start
• Mixing node order after insertion

4. Identify the error in the following code that inserts a node at the beginning of a circular linked list:

void insertAtBeginning(struct Node** head, int data) {
  struct Node* new_node = malloc(sizeof(struct Node));
  new_node->data = data;
  new_node->next = *head;
  *head = new_node;
}

medium

A. The head pointer is not updated to new node.

B. The new node's data is not assigned correctly.

C. Memory allocation is missing for new node.

D. The last node's next pointer is not updated to new head, breaking circular link.

Solution

1. Step 1: Check pointer updates

   The code updates head to new node and new node points to old head, but does not update last node's next pointer.

2. Step 2: Understand circular link maintenance

   In circular linked list, last node must point to new head to keep circle intact; missing here causes broken link.

3. Final Answer:

   The last node's next pointer is not updated to new head, breaking circular link. -> Option D

4. Quick Check:

   Last node must point to new head [OK]

Hint: Always update last node's next to new head [OK]

Common Mistakes:

• Ignoring last node pointer update
• Assuming head update is enough
• Not handling empty list case

5. You have a circular linked list with nodes: 5 -> 15 -> 25 -> back to 5. You want to insert 1 at the beginning. Which sequence of pointer updates correctly maintains the circular linked list?

hard

A. Create new node with 1, set last->next = new_node, new_node->next = head, head = new_node.

B. Create new node with 1, set head = new_node, new_node->next = head, update last->next = new_node.

C. Create new node with 1, set new_node->next = NULL, head = new_node, last->next = head.

D. Create new node with 1, set head = new_node, new_node->next = last, last->next = head.

Solution

1. Step 1: Link new node between last and head

   Set last->next to new node, then new_node->next to old head to maintain circle.

2. Step 2: Update head pointer

   Set head to new node so it becomes the first node in the list.

3. Final Answer:

   Create new node with 1, set last->next = new_node, new_node->next = head, head = new_node. -> Option A

4. Quick Check:

   Last points new node, new node points old head, head updated [OK]

Hint: Last points new node, new node points old head, update head last [OK]

Common Mistakes:

• Setting new_node->next to NULL breaks circle
• Updating head before linking last node
• Confusing pointer order causing broken links