Circular vs Linear Linked List Key Difference in DSA C - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a linked list is circular by comparing the next pointer of the last node.

DSA C

if (last->next == [1]) {
    printf("List is circular\n");
} else {
    printf("List is linear\n");
}

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

Atemp

BNULL

Clast

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if last->next is NULL instead of head.

Confusing the last node pointer with the head pointer.

In a circular linked list, the last node's next pointer points back to the head node.

2fill in blank

medium

Complete the code to traverse a circular linked list until it reaches the head again.

DSA C

struct Node* temp = head;
do {
    printf("%d -> ", temp->data);
    temp = temp->[1];
} while (temp != head);

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

Anext

Bchild

Cparent

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev pointer which may not exist in singly linked lists.

Using incorrect pointer names.

To traverse a linked list, we move from one node to the next using the next pointer.

3fill in blank

hard

Fix the error in the code that tries to detect a linear linked list by checking if next pointer is NULL.

DSA C

while (temp != NULL) {
    printf("%d -> ", temp->data);
    temp = temp->[1];
}
printf("NULL\n");

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

Ahead

Bprev

Cnext

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev pointer which may not exist.

Using head or tail pointers incorrectly.

To move through a linear linked list, we use the next pointer until it becomes NULL.

4fill in blank

hard

Fill both blanks to create a function that inserts a new node at the end of a circular linked list.

DSA C

void insertEnd(struct Node** head_ref, int data) {
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
    new_node->data = data;
    if (*head_ref == NULL) {
        new_node->next = new_node;
        *head_ref = new_node;
        return;
    }
    struct Node* temp = *head_ref;
    while (temp->[1] != *head_ref) {
        temp = temp->[2];
    }
    temp->next = new_node;
    new_node->next = *head_ref;
}

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

Anext

Bprev

Chead

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev pointer which may not exist.

Using incorrect pointer names.

To find the last node in a circular linked list, we move using the next pointer until we reach the head again.

5fill in blank

hard

Fill all three blanks to create a function that deletes the head node of a circular linked list.

DSA C

void deleteHead(struct Node** head_ref) {
    if (*head_ref == NULL) return;
    if ((*head_ref)->next == *head_ref) {
        free(*head_ref);
        *head_ref = NULL;
        return;
    }
    struct Node* temp = *head_ref;
    while (temp->[1] != *head_ref) {
        temp = temp->[2];
    }
    struct Node* to_delete = *head_ref;
    temp->next = (*head_ref)->[3];
    *head_ref = (*head_ref)->next;
    free(to_delete);
}

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

Anext

Bprev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev pointer which may not exist.

Not updating the last node's next pointer.

To delete the head in a circular linked list, find the last node using next pointers, then update its next to the second node.

Practice

1. What is the main difference between a circular linked list and a linear linked list?

easy

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

B. In a circular linked list, nodes are stored in an array.

C. A linear linked list has no nodes.

D. A linear linked list always has exactly two nodes.

Solution

1. Step 1: Understand the structure of a linear linked list

   A linear linked list has nodes connected one after another, and the last node points to null, ending the list.

2. Step 2: Understand the structure of a circular linked list

   In a circular linked list, the last node points back to the first node, creating a loop or circle.

3. Final Answer:

   In a circular linked list, the last node points back to the first node, forming a circle. -> Option A

4. Quick Check:

   Circular linked list loops back = In a circular linked list, the last node points back to the first node, forming a circle. [OK]

Hint: Circular lists loop back to start; linear lists end with null [OK]

Common Mistakes:

• Thinking circular lists use arrays
• Believing linear lists have no nodes
• Confusing last node pointers

2. Which of the following is the correct way to check if a circular linked list has reached back to the start node in C?

easy

A. if (current->next == NULL) { /* end */ }

B. if (current->next == head) { /* back to start */ }

C. if (current == NULL) { /* end */ }

D. if (current == head) { /* back to start */ }

Solution

1. Step 1: Understand circular linked list traversal

   In a circular linked list, traversal starts from head and continues until the current pointer returns to head, completing the loop.

2. Step 2: Identify the correct condition to detect reaching start

   if (current == head) checks if we have reached back to the start node after full traversal.

3. Final Answer:

   if (current == head) { /* back to start */ } -> Option D

4. Quick Check:

   current == head = reached back to start [OK]

Hint: Check if current == head to detect back to start [OK]

Common Mistakes:

• Using NULL to detect end in circular list
• Checking current->next == head (detects last node)
• Confusing current and current->next

3. Consider the following C code for traversing a circular linked list starting at head:

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");
}

If the list has nodes with data 1, 2, 3 linked circularly, what is the output?

medium

A. 1 -> 2 -> 3 -> 1 -> 2 -> 3 -> NULL

B. NULL

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

D. 1 -> 2 -> 3

Solution

1. Step 1: Understand the traversal loop

   The do-while loop prints the current node's data, then moves to next node, repeating until it reaches back to head.

2. Step 2: Trace the output for nodes 1, 2, 3

   It prints 1, then 2, then 3, then stops because temp == head again, then prints NULL.

3. Final Answer:

   1 -> 2 -> 3 -> NULL -> Option C

4. Quick Check:

   Prints all nodes once, then NULL [OK]

Hint: do-while prints all nodes once in circular list [OK]

Common Mistakes:

• Expecting infinite loop output
• Missing NULL at end
• Confusing do-while with while loop

4. The following code attempts to traverse a circular linked list but causes an infinite loop. What is the error?

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

medium

A. The printf statement is missing a newline.

B. The loop condition should check if temp equals head, not NULL.

C. The code should use a for loop instead of while.

D. The temp pointer is not initialized.

Solution

1. Step 1: Analyze the loop condition for circular list

   In a circular linked list, temp never becomes NULL because last node points back to head, so while(temp != NULL) never ends.

2. Step 2: Correct the loop condition to stop at head

   We should stop when temp reaches head again to avoid infinite loop.

3. Final Answer:

   The loop condition should check if temp equals head, not NULL. -> Option B

4. Quick Check:

   Stop at head, not NULL, in circular list [OK]

Hint: In circular lists, stop when back at head, not NULL [OK]

Common Mistakes:

• Using NULL to end circular list traversal
• Ignoring infinite loop possibility
• Not initializing temp properly

5. You want to convert a linear linked list into a circular linked list in C. Which code snippet correctly achieves this assuming head is the start node and the list is non-empty?

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

medium

A. temp->next = head; // link last node to head to form circle

B. head = NULL; // reset head to null

C. temp = NULL; // set temp to null to end list

D. temp->next = NULL; // keep list linear

Solution

1. Step 1: Find the last node in the linear list

   The while loop moves temp to the last node where next is NULL.

2. Step 2: Link last node's next to head to form circular list

   Setting temp->next = head connects the last node back to the first node, making the list circular.

3. Final Answer:

   temp->next = head; // link last node to head to form circle -> Option A

4. Quick Check:

   Last node points to head to make circle [OK]

Hint: Link last node's next to head to make circular [OK]

Common Mistakes:

• Setting head to NULL mistakenly
• Leaving last node's next as NULL
• Setting temp to NULL instead of linking