Delete from Beginning of Doubly Linked List in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after deleting the first node from a doubly linked list

What is the printed state of the doubly linked list after deleting the first node?

DSA C

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

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

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

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

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

    head->data = 10; head->prev = NULL; head->next = second;
    second->data = 20; second->prev = head; second->next = third;
    third->data = 30; third->prev = second; third->next = NULL;

    head = deleteBeginning(head);
    printList(head);
    return 0;
}

A NULL

B 10 <-> 20 <-> 30 <-> NULL

C 20 <-> 30 <-> NULL

D 30 <-> NULL

💡 Hint

Deleting the first node removes the node with data 10 and updates the head.

The function deletes the first node (10), updates head to second node (20), and sets its prev to NULL. The list becomes 20 <-> 30 <-> NULL.

❓ Predict Output

intermediate

Output when deleting from an empty doubly linked list

What is the output when trying to delete the first node from an empty doubly linked list?

DSA C

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

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

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

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

int main() {
    Node* head = NULL;
    head = deleteBeginning(head);
    printList(head);
    return 0;
}

A 0 <-> NULL

B NULL

C Segmentation fault

D List is empty

💡 Hint

Check if the list is empty before deleting.

Since the list is empty (head is NULL), deleteBeginning returns NULL and printList prints 'List is empty'.

🔧 Debug

advanced

Identify the bug in deleting the first node of a doubly linked list

What is the bug in the following deleteBeginning function for a doubly linked list?

DSA C

Node* deleteBeginning(Node* head) {
    if (head == NULL) return NULL;
    Node* temp = head;
    head = head->next;
    head->prev = NULL;
    free(temp);
    return head;
}

A It leaks memory by not freeing any node

B It causes a segmentation fault if the list has only one node

C It does not update the head pointer

D It frees the wrong node

💡 Hint

Consider what happens when head->next is NULL.

If the list has only one node, head->next is NULL, so head->prev = NULL causes a segmentation fault because head is NULL after assignment.

🧠 Conceptual

advanced

Effect of deleting the first node on the doubly linked list pointers

After deleting the first node from a doubly linked list, which pointers must be updated to maintain list integrity?

A The head pointer and the prev pointer of the new head node

B Only the head pointer

C Only the next pointer of the old head node

D The tail pointer and the next pointer of the new head node

💡 Hint

Think about what changes when the first node is removed.

The head pointer must point to the new first node, and the new head's prev pointer must be set to NULL to indicate it has no previous node.

❓ Predict Output

expert

Output after multiple deletions from the beginning of a doubly linked list

What is the printed state of the doubly linked list after deleting the first node twice in a row?

DSA C

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

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

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

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

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

    head->data = 5; head->prev = NULL; head->next = second;
    second->data = 15; second->prev = head; second->next = third;
    third->data = 25; third->prev = second; third->next = NULL;

    head = deleteBeginning(head);
    head = deleteBeginning(head);
    printList(head);
    return 0;
}

A 25 <-> NULL

B 15 <-> 25 <-> NULL

C 5 <-> 15 <-> 25 <-> NULL

D NULL

💡 Hint

Deleting twice removes first two nodes.

First deletion removes node with data 5, second removes node with data 15, leaving node with data 25 as head.

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