Insert at Beginning Head Insert in DSA C - Build from Scratch

Insert at Beginning (Head Insert) in a Linked List

📖 Scenario: Imagine you are managing a guest list for a party. New guests arrive and you want to add each new guest to the front of the list so they are greeted first.

🎯 Goal: You will build a simple linked list in C and learn how to insert new nodes at the beginning (head) of the list.

📋 What You'll Learn

Create a linked list node structure with an integer data field and a next pointer

Create a pointer head to represent the start of the list

Write a function insertAtBeginning to add a new node at the start

Insert three nodes with values 10, 20, and 30 at the beginning in that order

Print the linked list after all insertions showing the order of nodes

💡 Why This Matters

🌍 Real World

Linked lists are used in many applications like managing playlists, undo functionality in apps, and dynamic memory management.

💼 Career

Understanding linked lists and how to insert nodes is fundamental for software developers working with data structures and memory management.

1

Create the linked list node structure and initialize head

Write the struct definition for a linked list node named Node with an int data and a Node* pointer named next. Then create a Node* variable called head and initialize it to NULL.

DSA C

# Define the Node struct and initialize head
// Your code here

Hint

Use typedef struct Node { int data; struct Node* next; } Node; and set head to NULL.

2

Create the insertAtBeginning function

Write a function called insertAtBeginning that takes an int value parameter. Inside, create a new Node* using malloc, set its data to value, set its next to the current head, then update head to this new node.

DSA C

# Write insertAtBeginning function
// Your code here

Hint

Allocate memory for a new node, set its data, point its next to head, then update head.

3

Insert nodes with values 10, 20, and 30 at the beginning

Call the insertAtBeginning function three times to insert the values 10, 20, and 30 in that order.

DSA C

// Insert 10, 20, and 30 at the beginning
// Your code here

Hint

Call insertAtBeginning three times with 10, 20, and 30.

4

Print the linked list to show the order of nodes

Write a printList function that starts from head and prints each node's data followed by ->. After printing all nodes, print NULL. Then call printList() in main after the insertions.

DSA C

# Write printList function and call it in main
// Your code here

Hint

Traverse from head, print each node's data followed by '->', then print 'NULL'.

Practice

1. What does the "Insert at Beginning" operation do in a singly linked list?

easy

A. Deletes the first node of the list

B. Adds a new node at the end of the list

C. Adds a new node at the start of the list

D. Deletes the last node of the list

Solution

1. Step 1: Understand the operation

   Inserting at the beginning means placing a new node before the current first node.

2. Step 2: Effect on the list

   The new node becomes the head, pushing the old head to second position.

3. Final Answer:

   Adds a new node at the start of the list -> Option C

4. Quick Check:

   Insert at beginning = Add new head [OK]

Hint: New node becomes head, old head moves next [OK]

Common Mistakes:

• Confusing insert at beginning with insert at end
• Thinking it deletes nodes
• Assuming it inserts in the middle

2. Which of the following is the correct syntax to insert a new node at the beginning in C for a singly linked list?

easy

A. newNode->next = head; head = newNode;

B. head = newNode->next; newNode = head;

C. head->next = newNode; newNode = head;

D. newNode = head; head->next = newNode;

Solution

1. Step 1: Assign newNode's next to current head

   This links the new node to the existing list.

2. Step 2: Update head to newNode

   Now the new node is the first node in the list.

3. Final Answer:

   newNode->next = head; head = newNode; -> Option A

4. Quick Check:

   Link newNode to head, then update head [OK]

Hint: Point newNode->next to head, then head to newNode [OK]

Common Mistakes:

• Swapping assignments order
• Assigning head to newNode->next incorrectly
• Not updating head pointer

3. What will be the linked list after inserting nodes with values 10, then 20, then 30 at the beginning in that order? Assume the list was initially empty.

medium

A. 10 -> 20 -> 30 -> null

B. 10 -> 30 -> 20 -> null

C. 20 -> 30 -> 10 -> null

D. 30 -> 20 -> 10 -> null

Solution

1. Step 1: Insert 10 at beginning

   List becomes 10 -> null.

2. Step 2: Insert 20 at beginning

   List becomes 20 -> 10 -> null.

3. Step 3: Insert 30 at beginning

   List becomes 30 -> 20 -> 10 -> null.

4. Final Answer:

   30 -> 20 -> 10 -> null -> Option D

5. Quick Check:

   Last inserted is new head [OK]

Hint: Last inserted node is always head [OK]

Common Mistakes:

• Assuming insertion at end
• Reversing insertion order
• Confusing node values order

4. Given the following code snippet, what is the error that prevents correct insertion at the beginning?

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

void insertAtBeginning(struct Node** head, int val) {
  struct Node* newNode = malloc(sizeof(struct Node));
  newNode->data = val;
  newNode->next = NULL;
  *head = newNode;
}

medium

A. malloc size is incorrect

B. newNode->next should point to current head before updating head

C. newNode->data is not assigned

D. head pointer should not be double pointer

Solution

1. Step 1: Check newNode->next assignment

   It is set to NULL, losing the existing list.

2. Step 2: Correct linking

   newNode->next should point to current head before head is updated.

3. Final Answer:

   newNode->next should point to current head before updating head -> Option B

4. Quick Check:

   Link newNode->next to old head before head update [OK]

Hint: Link newNode->next to old head before head update [OK]

Common Mistakes:

• Setting newNode->next to NULL always
• Not using double pointer for head
• Forgetting to assign data

5. You have a singly linked list with nodes: 5 -> 15 -> 25 -> null. You want to insert a new node with value 10 at the beginning. After insertion, what will be the correct sequence of nodes?

hard

A. 10 -> 5 -> 15 -> 25 -> null

B. 5 -> 10 -> 15 -> 25 -> null

C. 10 -> 15 -> 25 -> 5 -> null

D. 5 -> 15 -> 25 -> 10 -> null

Solution

1. Step 1: Current list is 5 -> 15 -> 25 -> null

   This is the starting list before insertion.

2. Step 2: Insert 10 at beginning

   New node 10 points to current head (5), then head updated to 10.

3. Final Answer:

   10 -> 5 -> 15 -> 25 -> null -> Option A

4. Quick Check:

   New node becomes head, old head follows [OK]

Hint: New node points to old head, becomes new head [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating head pointer
• Linking nodes incorrectly