Delete from Beginning of Doubly Linked List in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the list is empty before deletion.

DSA C

if (head == [1]) {
    printf("List is empty\n");
    return;
}

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

Ahead->next

BNULL

Chead

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Checking head->next instead of head.

Using 0 instead of NULL for pointer comparison.

We check if head is NULL to know if the list is empty.

2fill in blank

medium

Complete the code to move the head pointer to the next node after deletion.

DSA C

struct Node* temp = head;
head = head->[1];
free(temp);

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Aprev

Bdata

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' to move head.

Not freeing the old head node.

After deleting the first node, head should point to the next node.

3fill in blank

hard

Fix the error in setting the previous pointer of the new head node to NULL.

DSA C

if (head != NULL) {
    head->[1] = NULL;
}

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Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 'next' to NULL instead of 'prev'.

Not checking if head is NULL before setting pointer.

The previous pointer of the new head should be set to NULL.

4fill in blank

hard

Fill both blanks to complete the function that deletes the first node of the doubly linked list.

DSA C

void deleteBeginning(struct Node** head_ref) {
    if (*head_ref == [1]) {
        return;
    }
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->[2];
    if (*head_ref != NULL) {
        (*head_ref)->prev = NULL;
    }
    free(temp);
}

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ANULL

Bhead

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' to move head.

Not checking if *head_ref is NULL before deletion.

We check if *head_ref is NULL to see if list is empty, then move head to next node.

5fill in blank

hard

Fill all three blanks to complete the main function that creates a list and deletes the first node.

DSA C

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

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

void deleteBeginning(struct Node** head_ref) {
    if (*head_ref == NULL) {
        return;
    }
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->next;
    if (*head_ref != NULL) {
        (*head_ref)->prev = NULL;
    }
    free(temp);
}

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

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

    head->data = 1;
    head->prev = NULL;
    head->next = second;

    second->data = 2;
    second->prev = head;
    second->next = third;

    third->data = 3;
    third->prev = second;
    third->next = NULL;

    deleteBeginning(&head);

    printf("List after deletion: ");
    struct Node* current = head;
    while (current != NULL) {
        printf("%d ", current->[1]);
        current = current->[2];
    }
    printf("\n");

    free(second);
    free(third);

    return [3];
}

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Adata

Bnext

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Printing 'next' or 'prev' instead of 'data'.

Returning 1 instead of 0 from main.

Using wrong pointer to traverse the list.

Print node data, move to next node, and return 0 from main.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It remains unchanged

B. It becomes NULL immediately

C. It moves to the second node in the list

D. It points to the last node

Solution

1. Step 1: Understand the head pointer role

   The head pointer points to the first node in the doubly linked list.

2. Step 2: Effect of deleting the first node

   When the first node is deleted, the head pointer must move to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option C

4. Quick Check:

   Head points to new first node after deletion [OK]

Hint: Head always moves to next node after deleting first [OK]

Common Mistakes:

• Assuming head becomes NULL even if list has more nodes
• Not updating head pointer after deletion
• Pointing head to last node mistakenly

2. Which of the following is the correct syntax to delete the first node in a doubly linked list in C?

easy

A. temp = head; head = head->next; free(temp); head->prev = NULL;

B. free(head); head = head->next; head->prev = NULL;

C. head = head->next; free(head->prev); head->prev = NULL;

D. head->prev = NULL; free(head); head = head->next;

Solution

1. Step 1: Store current head in temp

   We save the current first node in a temporary pointer to free it later.

2. Step 2: Move head to next node and free old head

   Update head to point to the second node, free the old first node, and set new head's prev to NULL.

3. Final Answer:

   temp = head; head = head->next; free(temp); head->prev = NULL; -> Option A

4. Quick Check:

   Proper order: save, move, free, update pointers [OK]

Hint: Save old head before freeing, then update pointers [OK]

Common Mistakes:

• Freeing head before moving pointer
• Not setting new head's prev to NULL
• Accessing freed memory after free

3. Given the doubly linked list: 10 <-> 20 <-> 30 <-> NULL, what will be the list after deleting the first node?

medium

A. 20 <-> 30 <-> NULL

B. 10 <-> 30 <-> NULL

C. 30 <-> NULL

D. NULL

Solution

1. Step 1: Identify the first node to delete

   The first node has value 10, which will be removed.

2. Step 2: Update head to second node and adjust pointers

   Head moves to node with value 20, and its prev pointer is set to NULL.

3. Final Answer:

   20 <-> 30 <-> NULL -> Option A

4. Quick Check:

   First node removed, list starts at 20 [OK]

Hint: After deletion, list starts from second node [OK]

Common Mistakes:

• Leaving old head in list
• Not updating prev pointer of new head
• Deleting wrong node

4. What is wrong with this code snippet for deleting the first node of a doubly linked list?

void deleteBegin() {
  if (head == NULL) return;
  free(head);
  head = head->next;
  if (head != NULL) head->prev = NULL;
}

medium

A. It does not free the node

B. It does not check if list is empty

C. It forgets to set head->prev to NULL

D. It frees head before moving pointer, causing invalid access

Solution

1. Step 1: Analyze order of operations

   The code frees head first, then tries to access head->next, which is invalid after free.

2. Step 2: Correct order needed

   We must first save head->next in a temp pointer, move head, then free old node to avoid invalid memory access.

3. Final Answer:

   It frees head before moving pointer, causing invalid access -> Option D

4. Quick Check:

   Free after pointer update to avoid invalid access [OK]

Hint: Always move pointer before freeing old node [OK]

Common Mistakes:

• Freeing node before updating pointers
• Accessing freed memory
• Not handling empty list properly

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> NULL. After deleting the first node twice, what will be the list?

hard

A. 15 <-> 25 <-> 35 <-> NULL

B. 25 <-> 35 <-> NULL

C. 35 <-> NULL

D. NULL

Solution

1. Step 1: Delete first node once

   First deletion removes node with value 5, head moves to 15.

2. Step 2: Delete first node again

   Second deletion removes node with value 15, head moves to 25.

3. Final Answer:

   25 <-> 35 <-> NULL -> Option B

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head to next node [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head correctly after second deletion
• Confusing node values after deletion