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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after inserting a node at the end of a circular linked list

What is the printed state of the circular linked list after inserting a node with value 4 at the end?

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 node at end */
void insertEnd(Node** head, int data) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = data;
    if (*head == NULL) {
        newNode->next = newNode;
        *head = newNode;
        return;
    }
    Node* temp = *head;
    while (temp->next != *head) {
        temp = temp->next;
    }
    temp->next = newNode;
    newNode->next = *head;
}

/* Function to print 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 = NULL;
    insertEnd(&head, 1);
    insertEnd(&head, 2);
    insertEnd(&head, 3);
    insertEnd(&head, 4);
    printList(head);
    return 0;
}

A 4 -> 1 -> 2 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 1 -> 2 -> 3 -> 4 -> null

D 1 -> 2 -> 3 -> 4

💡 Hint

Remember that in a circular linked list, the last node points back to the head, so printing stops when we reach the head again.

The insertEnd function adds the new node after the last node and points it back to the head. So the list becomes 1 -> 2 -> 3 -> 4 -> back to 1. The printList function prints nodes until it reaches the head again, so the output shows all four nodes followed by null.

❓ Predict Output

intermediate

Output after inserting into an empty circular linked list

What is the printed state of the circular linked list after inserting a node with value 10 into an empty list?

DSA C

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

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

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

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;
    insertEnd(&head, 10);
    printList(head);
    return 0;
}

A null

B 10 -> null

C 10 -> 10 -> null

D Error: segmentation fault

💡 Hint

When the list is empty, the new node points to itself.

Since the list is empty, the new node's next points to itself, making a single-node circular list. The printList function prints the node once and then stops when it reaches the head again.

🔧 Debug

advanced

Identify the error in this insertEnd function for circular linked list

What error will this code cause when inserting a node at the end of a circular linked list?

DSA C

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

A Infinite loop because temp->next never becomes NULL in circular list

B Segmentation fault due to dereferencing NULL pointer

C Correct insertion with no errors

D Memory leak due to missing free

💡 Hint

In a circular linked list, the last node's next points to head, not NULL.

The while loop condition checks for temp->next != NULL, but in a circular linked list, the last node's next points to head, never NULL. So the loop never ends, causing an infinite loop.

❓ Predict Output

advanced

Output after multiple insertions at end in circular linked list

What is the printed state of the circular linked list after these insertions: 5, 10, 15, 20?

DSA C

#include <stdio.h>
#include <stdlib.h>

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

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

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;
    insertEnd(&head, 5);
    insertEnd(&head, 10);
    insertEnd(&head, 15);
    insertEnd(&head, 20);
    printList(head);
    return 0;
}

A 5 -> 10 -> 15 -> 20

B 20 -> 15 -> 10 -> 5 -> null

C 5 -> 10 -> 15 -> null

D 5 -> 10 -> 15 -> 20 -> null

💡 Hint

Nodes are inserted at the end, so order is preserved.

Each new node is added after the last node and points back to head. The printList prints all nodes in insertion order followed by null.

🧠 Conceptual

expert

Why is the last node's next pointer important in a circular linked list?

What is the main reason the last node in a circular linked list points back to the head node?

A To make traversal continuous without needing a NULL check

B To mark the end of the list with a NULL pointer

C To store the size of the list in the last node

D To prevent insertion at the end of the list

💡 Hint

Think about how you know when to stop traversing a normal linked list.

In a circular linked list, the last node points back to the head to allow continuous traversal without encountering a NULL pointer. This makes the list circular and traversal can loop indefinitely.

Practice

1. What happens when you insert a new node at the end of a circular linked list that currently has 3 nodes?

easy

A. The new node becomes the head node and points to the second node.

B. The new node becomes the last node and points back to the head node.

C. The new node is inserted in the middle of the list.

D. The list breaks and becomes a linear linked list.

Solution

1. Step 1: Understand circular linked list structure

   In a circular linked list, the last node points back to the head node, forming a circle.

2. Step 2: Insert new node at end

   The new node is linked after the current last node, and its next pointer points to the head node, maintaining the circle.

3. Final Answer:

   The new node becomes the last node and points back to the head node. -> Option B

4. Quick Check:

   New node points to head, preserving circular link [OK]

Hint: New end node always points to head in circular list [OK]

Common Mistakes:

• Forgetting to link new node back to head
• Inserting node at wrong position
• Breaking the circular link

2. Which of the following is the correct syntax to create a new node in C for a circular linked list?

easy

A. struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

B. newNode = malloc(sizeof(Node));

C. struct Node newNode = malloc(sizeof(struct Node));

D. Node* newNode = new Node();

Solution

1. Step 1: Allocate memory for new node

   In C, use malloc with sizeof(struct Node) and cast to (struct Node*).

2. Step 2: Correct pointer declaration

   Declare newNode as a pointer to struct Node to hold the allocated memory address.

3. Final Answer:

   struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); -> Option A

4. Quick Check:

   Proper malloc and pointer cast in C [OK]

Hint: Use malloc with sizeof(struct Node) and cast to struct Node* [OK]

Common Mistakes:

• Missing (struct Node*) cast
• Declaring newNode as struct Node instead of pointer
• Using C++ style new operator in C

3. Given the following code snippet for inserting at the end of a circular linked list, what will be the printed list after insertion?

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

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

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

  head->data = 1; head->next = second;
  second->data = 2; second->next = third;
  third->data = 3; third->next = head;

  // Insert new node with data 4 at end
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
  newNode->data = 4;
  newNode->next = head;
  third->next = newNode;

  printList(head);
  return 0;
}

medium

A. 1 -> 2 -> 3 -> 4 -> 1 -> ... (infinite loop)

B. 1 -> 2 -> 3 -> NULL

C. 3 -> 4 -> 1 -> 2 -> NULL

D. 1 -> 2 -> 3 -> 4 -> NULL

Solution

1. Step 1: Understand list before insertion

   Initial list is 1 -> 2 -> 3 -> back to 1 (circular).

2. Step 2: Insert new node at end

   New node with data 4 points to head; third node points to new node, extending list to 1 -> 2 -> 3 -> 4 -> back to 1.

3. Step 3: Print list

   printList prints nodes until it reaches head again, so output is 1 -> 2 -> 3 -> 4 -> NULL.

4. Final Answer:

   1 -> 2 -> 3 -> 4 -> NULL -> Option D

5. Quick Check:

   New node added at end, circular link maintained [OK]

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

Common Mistakes:

• Printing infinite loop by not stopping at head
• Forgetting to update last node's next pointer
• Incorrect print format

4. Identify the error in the following code snippet that attempts to insert a new node at the end of a circular linked list:

void insertEnd(struct Node** head, int data) {
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
  newNode->data = data;
  newNode->next = *head;

  struct Node* temp = *head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  temp->next = newNode;
}

medium

A. The function should not use double pointer for head.

B. newNode->next should be NULL instead of *head.

C. The while loop condition should check for temp->next != *head instead of NULL.

D. Memory allocation for newNode is missing.

Solution

1. Step 1: Analyze loop condition in circular list

   In a circular linked list, last node's next points to head, not NULL.

2. Step 2: Correct loop condition

   Loop should continue until temp->next equals head, not NULL, to find last node.

3. Final Answer:

   The while loop condition should check for temp->next != *head instead of NULL. -> Option C

4. Quick Check:

   Loop must detect circular end by comparing to head [OK]

Hint: In circular list, last node points to head, not NULL [OK]

Common Mistakes:

• Using NULL to detect end in circular list
• Setting newNode->next to NULL
• Not updating last node's next pointer

5. You have a circular linked list with only one node where head points to itself. You want to insert a new node at the end. Which sequence of steps correctly updates the list?

hard

A. Create new node, set newNode->next = head->next, set head->next = newNode.

B. Create new node, set newNode->next = head, set head->next = newNode, then update head to newNode.

C. Create new node, set newNode->next = NULL, set head->next = newNode.

D. Create new node, set newNode->next = newNode, set head = newNode.

Solution

1. Step 1: Understand single node circular list

   The single node has head->next pointing to itself (head).

2. Step 2: Insert at end

   Set newNode->next = head->next (which is head), then head->next = newNode. This makes head -> newNode -> head, maintaining circularity without changing head.

3. Final Answer:

   Create new node, set newNode->next = head->next, set head->next = newNode. -> Option A

4. Quick Check:

   head -> newNode -> head [OK]

Hint: For single node circular: newNode->next = head->next; head->next = newNode [OK]

Common Mistakes:

• Setting newNode->next to NULL breaking circularity
• Unnecessarily updating head to newNode
• Incorrectly linking new node to itself