Insert at Beginning of Doubly Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows traversal in both directions.

beginner

What steps are needed to insert a new node at the beginning of a doubly linked list?

1. Create a new node.
2. Set new node's next pointer to current head.
3. Set new node's previous pointer to NULL.
4. If list is not empty, set current head's previous pointer to new node.
5. Update head to new node.

beginner

In C, what struct members are needed for a doubly linked list node?

A struct for a doubly linked list node typically has:
- an integer data field
- a pointer to the next node
- a pointer to the previous node

beginner

Why do we set the previous pointer of the new node to NULL when inserting at the beginning?

Because the new node will be the first node, it has no previous node, so its previous pointer must be NULL to mark the start of the list.

beginner

What happens if the doubly linked list is empty when inserting at the beginning?

If the list is empty, the new node becomes the only node. Its next and previous pointers are NULL, and the head points to this new node.

What pointer of the new node should point to the current head when inserting at the beginning?

A head

B next

C data

D previous

The new node's next pointer should point to the current head node.

When inserting at the beginning, what should the previous pointer of the new node be set to?

A itself

B head

C next node

D NULL

The previous pointer of the new node is NULL because it becomes the first node.

If the list is not empty, what pointer of the old head node must be updated after insertion?

A previous

B next

C data

D head

The old head node's previous pointer must point to the new node.

What is the final step after inserting a new node at the beginning?

A Update head to new node

B Delete old head

C Set new node's data to zero

D Set new node's next to NULL

The head pointer must be updated to point to the new node.

Which of these is NOT a part of a doubly linked list node?

A Data field

B Pointer to next node

C Pointer to head node

D Pointer to previous node

Nodes do not have a pointer to the head; only next and previous pointers.

Explain how to insert a new node at the beginning of a doubly linked list in C.

Think about pointers and updating head.

Describe the structure of a doubly linked list node and why each pointer is important.

Imagine walking forward and backward through a chain.

Practice

1. What is the main purpose of inserting a node at the beginning of a doubly linked list?

easy

A. To add a new node after the tail node

B. To add a new node before the current head node

C. To delete the first node of the list

D. To reverse the entire list

Solution

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

   A doubly linked list has nodes connected both forward and backward, with a head and tail.

2. Step 2: Identify what inserting at the beginning means

   Inserting at the beginning means adding a new node before the current head node, making it the new head.

3. Final Answer:

   To add a new node before the current head node -> Option B

4. Quick Check:

   Insert at beginning = add before head [OK]

Hint: Insert at beginning means new node becomes the head [OK]

Common Mistakes:

• Confusing insertion at beginning with insertion at end
• Thinking insertion deletes nodes
• Mixing up head and tail positions

2. Which of the following is the correct way to update the previous pointer of the old head node when inserting a new node at the beginning in C?

easy

A. new_node->prev = old_head;

B. old_head->next = new_node;

C. new_node->next = old_head->prev;

D. old_head->prev = new_node;

Solution

1. Step 1: Identify pointer updates when inserting at beginning

   The new node's next points to old head, and old head's prev points back to new node.

2. Step 2: Choose correct pointer update for old head

   We must set old_head->prev = new_node to link backward to new node.

3. Final Answer:

   old_head->prev = new_node; -> Option D

4. Quick Check:

   Old head prev points to new node [OK]

Hint: Old head's prev points to new node after insertion [OK]

Common Mistakes:

• Setting old_head->next instead of prev
• Assigning new_node->prev incorrectly
• Forgetting to update old head's prev pointer

3. What will be the output of the following code snippet after inserting a node with value 10 at the beginning of the doubly linked list?

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

Node* head = NULL;

void insertAtBeginning(int val) {
  Node* new_node = (Node*)malloc(sizeof(Node));
  new_node->data = val;
  new_node->prev = NULL;
  new_node->next = head;
  if (head != NULL) {
    head->prev = new_node;
  }
  head = new_node;
}

int main() {
  insertAtBeginning(20);
  insertAtBeginning(10);
  Node* temp = head;
  while (temp != NULL) {
    printf("%d ", temp->data);
    temp = temp->next;
  }
  return 0;
}

medium

A. 10

B. 20 10

C. 10 20

D. 20

Solution

1. Step 1: Trace first insertion insertAtBeginning(20)

   List is empty, new node with 20 becomes head: 20 -> NULL

2. Step 2: Trace second insertion insertAtBeginning(10)

   New node with 10 points next to old head (20), old head prev points to new node, head updated to 10: 10 <-> 20

3. Final Answer:

   10 20 -> Option C

4. Quick Check:

   List after insertions = 10 20 [OK]

Hint: New node becomes head, old head shifts right [OK]

Common Mistakes:

• Printing nodes in reverse order
• Not updating head pointer
• Forgetting to update old head's prev pointer

4. Identify the error in the following code snippet for inserting a node at the beginning of a doubly linked list:

void insertAtBeginning(int val) {
  Node* new_node = (Node*)malloc(sizeof(Node));
  new_node->data = val;
  new_node->prev = NULL;
  new_node->next = head;
  if (head != NULL) {
    head->next = new_node;
  }
  head = new_node;
}

medium

A. head->next should be head->prev when updating old head

B. new_node->prev should not be NULL

C. head pointer should not be updated

D. malloc size is incorrect

Solution

1. Step 1: Check pointer updates for old head

   When inserting at beginning, old head's prev pointer must point to new node, not next.

2. Step 2: Identify incorrect line

   Code uses head->next = new_node; which wrongly sets old head's next pointer instead of prev.

3. Final Answer:

   head->next should be head->prev when updating old head -> Option A

4. Quick Check:

   Old head prev must point to new node [OK]

Hint: Old head prev, not next, points to new node [OK]

Common Mistakes:

• Updating old head's next pointer instead of prev
• Not updating head pointer
• Incorrect malloc size usage

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25. You want to insert a new node with value 1 at the beginning. After insertion, what will be the correct sequence of node values when traversing from head to tail?

medium

A. 1 <-> 5 <-> 15 <-> 25

B. 5 <-> 15 <-> 25 <-> 1

C. 25 <-> 15 <-> 5 <-> 1

D. 1 <-> 15 <-> 5 <-> 25

Solution

1. Step 1: Understand insertion at beginning

   New node with value 1 becomes new head, pointing next to old head (5).

2. Step 2: Update pointers and traverse list

   Sequence after insertion: 1 (head) <-> 5 <-> 15 <-> 25 (tail).

3. Final Answer:

   1 <-> 5 <-> 15 <-> 25 -> Option A

4. Quick Check:

   New head is 1, followed by old nodes [OK]

Hint: New node becomes head, old head shifts right [OK]

Common Mistakes:

• Appending new node at end instead of beginning
• Mixing node order after insertion
• Not updating head pointer