Delete Node at Beginning in DSA C++ - Execution Trace

Concept Flow - Delete Node at Beginning

Start

↓

Check if list is empty

↓

Save head node

↓

Move head to next node

↓

Delete saved node

↓

Updated list

↓

End

This flow shows checking if the list is empty, then removing the first node by moving the head pointer and deleting the old first node.

Execution Sample

DSA C++

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

void deleteAtBeginning(Node*& head) {
  if (head == nullptr) return;
  Node* temp = head;
  head = head->next;
  delete temp;
}

This code deletes the first node of a singly linked list if it exists.

Execution Table

Step	Action	head (address)	head->data	temp (address)	List state
1	Start with list 10->20->30->null	0x100	10	null	10 -> 20 -> 30 -> null
2	Check if head is nullptr	0x100	10	null	List not empty, continue
3	Set temp = head	0x100	10	0x100	temp points to node with 10
4	Move head to head->next	0x110	20	0x100	head now points to node with 20
5	Delete temp node	0x110	20	deleted	List is 20 -> 30 -> null
6	End	0x110	20	deleted	Final list: 20 -> 30 -> null

💡 head moved to next node and old first node deleted, list updated

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
head	0x100 (10)	0x100 (10)	0x110 (20)	0x110 (20)	0x110 (20)
temp	null	0x100 (10)	0x100 (10)	deleted	deleted

Key Moments - 3 Insights

Why do we check if head is nullptr before deleting?

Because if head is nullptr, the list is empty and there is no node to delete. This is shown in execution_table step 2 where we skip deletion if head is nullptr.

Why do we save head in temp before moving head?

We save head in temp to keep a reference to the node to delete after moving head to the next node. This is shown in step 3 and 4 of execution_table.

What happens to the deleted node's memory?

The deleted node's memory is freed using delete, preventing memory leaks. This is shown in step 5 where temp is deleted.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of head->data after step 4?

A 20

B 10

C 30

D nullptr

💡 Hint

Check the 'head->data' column at step 4 in execution_table.

At which step is the first node actually deleted?

A Step 3

B Step 5

C Step 4

D Step 6

💡 Hint

Look for the action 'Delete temp node' in execution_table.

If the list was empty (head == nullptr), what would happen?

A The function crashes

B The function deletes a node anyway

C The function returns immediately without changes

D The function moves head to next node

💡 Hint

Refer to step 2 in execution_table where the check for nullptr happens.

Concept Snapshot

Delete Node at Beginning:
- Check if list is empty (head == nullptr)
- Save current head in temp
- Move head to next node
- Delete temp node to free memory
- Updated list starts from new head

Full Transcript

This concept shows how to delete the first node of a singly linked list. We first check if the list is empty by seeing if head is nullptr. If not empty, we save the current head node in a temporary pointer. Then we move head to the next node in the list. Finally, we delete the old head node using the temporary pointer to free memory. The list now starts from the new head node. This prevents memory leaks and updates the list correctly.

Practice

1. What happens when you delete the first node of a singly linked list?

easy

A. The head pointer moves to the second node, and the first node is removed.

B. The last node is deleted instead of the first.

C. The list becomes empty regardless of its size.

D. The second node is deleted, but the head remains the same.

Solution

1. Step 1: Understand the head pointer role

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

2. Step 2: Delete the first node by moving head

   To delete the first node, update head to point to the second node, removing the first node from the list.

3. Final Answer:

   The head pointer moves to the second node, and the first node is removed. -> Option A

4. Quick Check:

   Deleting first node means head moves forward [OK]

Hint: Deleting first node means head points to next node [OK]

Common Mistakes:

• Deleting the wrong node (not the first)
• Not updating the head pointer
• Assuming list becomes empty always
• Confusing first and second nodes

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

easy

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

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

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

D. Node* temp = head; head = head->next; delete temp;

Solution

1. Step 1: Store the current head node

   We save the current head in a temporary pointer to delete it safely later.

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

   Update head to point to the next node, then delete the old head using the temporary pointer.

3. Final Answer:

   Node* temp = head; head = head->next; delete temp; -> Option D

4. Quick Check:

   Save old head, move head, delete old head [OK]

Hint: Save old head before deleting to avoid losing access [OK]

Common Mistakes:

• Deleting head before moving pointer
• Not saving old head before deleting
• Assigning head incorrectly
• Deleting head twice

3. What will be the output linked list after executing the following code?

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

Node* head = new Node{10, new Node{20, new Node{30, nullptr}}};

Node* temp = head;
head = head->next;
delete temp;

Node* curr = head;
while(curr != nullptr) {
  std::cout << curr->data << " -> ";
  curr = curr->next;
}
std::cout << "null";

medium

A. 20 -> 30 -> null

B. null

C. 30 -> null

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

Solution

1. Step 1: Initial linked list setup

   The list starts as 10 -> 20 -> 30 -> null.

2. Step 2: Delete first node and update head

   We save head (10) in temp, move head to 20, then delete node 10.

3. Step 3: Print remaining list

   Starting from new head (20), we print 20 -> 30 -> null.

4. Final Answer:

   20 -> 30 -> null -> Option A

5. Quick Check:

   First node deleted, list starts at 20 [OK]

Hint: After deleting head, list starts from next node [OK]

Common Mistakes:

• Printing deleted node data
• Not updating head pointer
• Deleting wrong node
• Forgetting to print null at end

4. Identify the error in the following code snippet that deletes the first node of a linked list:

void deleteFirstNode(Node* head) {
  Node* temp = head;
  head = head->next;
  delete temp;
}

medium

A. head->next is not a valid pointer.

B. Deleting temp before moving head causes runtime error.

C. The function does not update the original head pointer outside the function.

D. No error; code works correctly.

Solution

1. Step 1: Understand pointer passing in C++

   Function receives head by value, so changes inside do not affect original head.

2. Step 2: Identify missing update to original head

   Updating local head pointer does not update the caller's head, so list head remains unchanged.

3. Final Answer:

   The function does not update the original head pointer outside the function. -> Option C

4. Quick Check:

   Pointer passed by value, original head unchanged [OK]

Hint: Pass head by reference to update original pointer [OK]

Common Mistakes:

• Assuming head changes outside function
• Deleting node before moving head
• Confusing pointer and value passing
• Ignoring nullptr checks

5. Given a singly linked list, write the correct approach to delete the first node safely and update the head pointer in C++. Which of the following code snippets correctly achieves this?

hard

A. void deleteFirstNode(Node* head) {
Node* temp = head;
head = head->next;
delete temp;
}

B. void deleteFirstNode(Node*& head) {
if (head == nullptr) return;
Node* temp = head;
head = head->next;
delete temp;
}

C. void deleteFirstNode(Node** head) {
Node* temp = *head;
*head = (*head)->next;
delete temp;
}

D. void deleteFirstNode(Node head) {
head = head.next;
delete head;
}

Solution

1. Step 1: Pass head by reference to modify original pointer

   Using Node*& head allows the function to update the caller's head pointer directly.

2. Step 2: Check for empty list and delete first node safely

   Check if head is nullptr to avoid errors, then move head to next node and delete old first node.

3. Final Answer:

   void deleteFirstNode(Node*& head) { if (head == nullptr) return; Node* temp = head; head = head->next; delete temp; } -> Option B

4. Quick Check:

   Reference parameter updates head safely [OK]

Hint: Use reference parameter to update head pointer [OK]

Common Mistakes:

• Passing head by value instead of reference
• Not checking for empty list
• Deleting node without updating head
• Using incorrect pointer syntax