Why Circular Linked List and Real World Use Cases in DSA C - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a circular linked list node that points to itself.

DSA C

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

struct Node* createNode(int value) {
    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = [1];
    return newNode;
}

Drag options to blanks, or click blank then click option'

AnewNode

BNULL

C0

D&newNode

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to NULL instead of pointing to the node itself.

Using the address of the pointer variable instead of the node.

In a circular linked list, the node's next pointer points to itself when it is the only node, so newNode->next = newNode; creates a circular link.

2fill in blank

medium

Complete the code to traverse a circular linked list starting from head and print all node data once.

DSA C

void printList(struct Node* head) {
    struct Node* temp = head;
    if (head == NULL) return;
    do {
        printf("%d -> ", temp->data);
        temp = [1];
    } while (temp != head);
    printf("NULL\n");
}

Drag options to blanks, or click blank then click option'

Ahead

Btemp

Chead->next

Dtemp->next

Attempts:

3 left

💡 Hint

Common Mistakes

Using head instead of temp->next, causing an infinite loop.

Not updating temp at all.

To move through the circular linked list, we update temp to temp->next in each iteration.

3fill in blank

hard

Fix the error in the code to insert a new node after the head in a circular linked list.

DSA C

void insertAfterHead(struct Node* head, int value) {
    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = head->next;
    head->next = [1];
}

Drag options to blanks, or click blank then click option'

Ahead

BNULL

CnewNode

Dhead->next

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning head->next to head instead of newNode.

Assigning NULL to head->next.

To insert the new node after head, set head->next = newNode; so the head points to the new node.

4fill in blank

hard

Fill both blanks to check if a circular linked list is empty or has only one node.

DSA C

int isSingleNode(struct Node* head) {
    if (head == NULL) return 0;
    if (head->next == [1] || head->next == [2]) return 1;
    return 0;
}

Drag options to blanks, or click blank then click option'

Ahead

BNULL

Chead->next

Dhead->data

Attempts:

3 left

💡 Hint

Common Mistakes

Checking head->next against NULL instead of head.

Using wrong pointers for comparison.

For a circular linked list, if head->next == head (only one node), we return 1. The check for NULL is not typical in circular lists as next should never be NULL.

5fill in blank

hard

Fill all three blanks to create a function that counts nodes in a circular linked list.

DSA C

int countNodes(struct Node* head) {
    if (head == NULL) return 0;
    int count = 1;
    struct Node* temp = head->[1];
    while (temp != [2]) {
        count++;
        temp = temp->[3];
    }
    return count;
}

Drag options to blanks, or click blank then click option'

Anext

Bhead

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using data instead of next to move to the next node.

Comparing temp to NULL instead of head.

Start from the node after head (head->next), loop until back to head, moving to temp->next each time, counting nodes.

Practice

1. What is the main advantage of using a circular linked list over a singly linked list?

easy

A. It automatically sorts the elements

B. It uses less memory than singly linked list

C. It allows continuous traversal from any node without reaching null

D. It prevents duplicate elements

Solution

1. Step 1: Understand traversal in singly linked list

   In a singly linked list, traversal stops when the next pointer is null, meaning the end of the list.

2. Step 2: Understand traversal in circular linked list

   In a circular linked list, the last node points back to the first node, allowing traversal to continue indefinitely without hitting null.

3. Final Answer:

   It allows continuous traversal from any node without reaching null -> Option C

4. Quick Check:

   Continuous traversal = Advantage of circular linked list [OK]

Hint: Circular list loops back to start, no null end [OK]

Common Mistakes:

• Thinking circular lists use less memory
• Assuming circular lists sort data automatically
• Confusing circular with doubly linked lists

2. Which of the following is the correct way to check if a circular linked list is empty in C?

easy

A. if (head == head->next) { /* empty */ }

B. if (head->next == NULL) { /* empty */ }

C. if (head->next == head) { /* empty */ }

D. if (head == NULL) { /* empty */ }

Solution

1. Step 1: Understand empty list condition

   An empty circular linked list has no nodes, so the head pointer is NULL.

2. Step 2: Analyze options

   if (head == NULL) { /* empty */ } correctly checks if head is NULL. Options A, B, and C assume head is not NULL and check next pointers, which is invalid if list is empty.

3. Final Answer:

   if (head == NULL) { /* empty */ } -> Option D

4. Quick Check:

   Empty list means head is NULL [OK]

Hint: Empty list always has head as NULL [OK]

Common Mistakes:

• Checking next pointer when head is NULL causes errors
• Confusing circular list empty check with non-circular
• Using head->next == head as empty condition

3. Consider the following C code snippet for a circular linked list traversal:

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");
}

What will be the output if the list has nodes with data 10, 20, 30 linked circularly?

medium

A. 10 -> 20 -> 30 -> NULL

B. 10 -> 20 -> 30 -> 10 -> ... (infinite loop)

C. NULL

D. 10 20 30

Solution

1. Step 1: Understand traversal logic

   The do-while loop prints current node data and moves to next until it reaches the head again, ensuring each node is printed once.

2. Step 2: Trace output for nodes 10, 20, 30

   Prints 10, then 20, then 30, then stops because temp == head again, then prints NULL.

3. Final Answer:

   10 -> 20 -> 30 -> NULL -> Option A

4. Quick Check:

   One full cycle print then NULL [OK]

Hint: do-while stops after one full circle [OK]

Common Mistakes:

• Thinking it loops infinitely
• Missing the NULL print at end
• Confusing do-while with while loop

4. The following code attempts to insert a new node at the end of a circular linked list. What is the bug?

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

medium

A. Does not update head when list is empty

B. New node's next pointer is not set correctly

C. Does not set new node as head when list is empty

D. Memory allocation is missing

Solution

1. Step 1: Analyze insertion when list is empty

   If *head is NULL, newNode is assigned to *head, but newNode->next points to *head which was NULL, so newNode->next is NULL, breaking circularity.

2. Step 2: Fix circular link for single node

   For a single node circular list, newNode->next should point to itself, not NULL, to maintain circular structure.

3. Final Answer:

   New node's next pointer is not set correctly -> Option B

4. Quick Check:

   Single node must point to itself [OK]

Hint: Single node next must point to itself [OK]

Common Mistakes:

• Forgetting to set newNode->next = newNode when empty
• Assuming malloc is missing
• Not updating last node's next pointer

5. Which real-world scenario best fits the use of a circular linked list?

hard

A. Implementing a music playlist that repeats songs endlessly

B. Storing a sorted list of student grades

C. Managing a stack of browser history

D. Representing a binary search tree

Solution

1. Step 1: Understand circular linked list use cases

   Circular linked lists are ideal for scenarios needing continuous looping through elements without stopping.

2. Step 2: Match options to use cases

   A music playlist that repeats songs endlessly fits perfectly because after the last song, it loops back to the first. Other options do not require circular traversal.

3. Final Answer:

   Implementing a music playlist that repeats songs endlessly -> Option A

4. Quick Check:

   Endless looping = Circular linked list use [OK]

Hint: Looping playlist = circular linked list use case [OK]

Common Mistakes:

• Choosing sorted list which needs order, not circular
• Confusing stack with circular list
• Thinking binary trees are circular