Delete Node at Beginning in DSA Python

Mental Model

To remove the first item, just move the start pointer to the next item, skipping the first.

Analogy: Imagine a line of people holding hands. To remove the first person, the second person becomes the new start of the line.

head -> 1 -> 2 -> 3 -> null

Dry Run Walkthrough

Input: list: 1 -> 2 -> 3, delete node at beginning

Goal: Remove the first node and update the list to start from the second node

Step 1: Set head to point to the second node

head -> [2] -> 3 -> null

Why: We skip the first node by moving head to the next node

Step 2: The first node (1) is no longer referenced and is removed

head -> 2 -> 3 -> null

Why: Without any pointer to node 1, it is effectively deleted

Result:

head -> 2 -> 3 -> null

Annotated Code

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = new_node
            return
        last = self.head
        while last.next:
            last = last.next
        last.next = new_node

    def delete_at_beginning(self):
        if not self.head:
            return  # List is empty, nothing to delete
        self.head = self.head.next  # Move head to next node, removing first

    def __str__(self):
        result = []
        current = self.head
        while current:
            result.append(str(current.data))
            current = current.next
        return ' -> '.join(result) + ' -> null'

# Driver code
ll = LinkedList()
ll.append(1)
ll.append(2)
ll.append(3)
print("Before deletion:", ll)
ll.delete_at_beginning()
print("After deletion:", ll)

if not self.head:

check if list is empty to avoid errors

self.head = self.head.next

advance head pointer to next node to remove first node

Output

Before deletion: 1 -> 2 -> 3 -> null

After deletion: 2 -> 3 -> null

Complexity Analysis

Time: O(1) because we only change the head pointer once

Space: O(1) because no extra space is used

vs Alternative: Compared to deleting a node in the middle which requires traversal O(n), deleting at beginning is faster and simpler

Edge Cases

empty list

No deletion occurs, list remains empty

DSA Python

if not self.head:

single node list

After deletion, list becomes empty (head is None)

DSA Python

self.head = self.head.next

When to Use This Pattern

When asked to remove the first element of a linked list quickly, use the delete at beginning pattern by moving the head pointer.

Common Mistakes

Mistake: Trying to delete the node by traversing or searching instead of just moving head
Fix: Directly assign head to head.next without traversal

Summary

Deletes the first node by moving the head pointer to the next node.

Use when you need to remove the first element of a linked list efficiently.

The key insight is that the head pointer controls the start of the list, so changing it removes the first node.

Practice

1. What happens to the head pointer when you delete a node at the beginning of a singly linked list?

easy

A. It points to the last node

B. It becomes null immediately

C. It remains unchanged

D. It moves to the next node in the list

Solution

1. Step 1: Understand the head pointer role

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

2. Step 2: Effect of deleting the first node

   Deleting the first node means the head must now point to the second node, which becomes the new first node.

3. Final Answer:

   It moves to the next node in the list -> Option D

4. Quick Check:

   Head points to next node after deletion [OK]

Hint: Head always moves to next node when deleting at start [OK]

Common Mistakes:

• Thinking head becomes null even if list has more nodes
• Assuming head points to last node after deletion
• Not updating head pointer at all

2. Which of the following is the correct Python syntax to delete the first node in a singly linked list with head pointer head?

easy

A. head = head.next

B. head.next = head

C. head = null

D. delete head

Solution

1. Step 1: Understand Python linked list node deletion

   To delete the first node, we move the head pointer to the next node.

2. Step 2: Check syntax correctness

   Assigning head = head.next correctly moves head to the next node. Other options are invalid Python or incorrect logic.

3. Final Answer:

   head = head.next -> Option A

4. Quick Check:

   Assign head to head.next to delete first node [OK]

Hint: Use head = head.next to delete first node [OK]

Common Mistakes:

• Using 'null' instead of 'None' in Python
• Trying to delete node with 'delete' keyword (not valid in Python)
• Assigning head.next = head which breaks list

3. What will be the output linked list after deleting the first node from the list: 1 -> 2 -> 3 -> 4 -> null?

medium

A. 3 -> 4 -> null

B. 1 -> 3 -> 4 -> null

C. 2 -> 3 -> 4 -> null

D. 1 -> 2 -> 3 -> 4 -> null

Solution

1. Step 1: Identify the first node to delete

   The first node has value 1.

2. Step 2: Delete first node and update head

   After deletion, head points to node with value 2, so list becomes 2 -> 3 -> 4 -> null.

3. Final Answer:

   2 -> 3 -> 4 -> null -> Option C

4. Quick Check:

   First node removed, list starts at 2 [OK]

Hint: Remove first node value, start list from second node [OK]

Common Mistakes:

• Removing wrong node (like second node)
• Not updating head pointer
• Assuming list becomes empty

4. Given the code below, what is the error when deleting the first node?

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

head = Node(10)
head.next = Node(20)
head = head.next.next  # Delete first node

medium

A. head becomes None causing loss of list

B. SyntaxError due to invalid assignment

C. No error, list updates correctly

D. RuntimeError due to NoneType access

Solution

1. Step 1: Trace the assignment head = head.next.next

   head.next is node with data 20, head.next.next is None (end of list).

2. Step 2: Effect of assignment

   Assigning head to None loses the entire list, effectively emptying it.

3. Final Answer:

   head becomes None causing loss of list -> Option A

4. Quick Check:

   head = head.next.next sets head to None [OK]

Hint: Assign head = head.next to delete first node, not head.next.next [OK]

Common Mistakes:

• Assigning head to head.next.next instead of head.next
• Expecting list to remain intact after wrong assignment
• Ignoring None assignment effect

5. You have a singly linked list with nodes: 5 -> 10 -> 15 -> 20 -> null. Write the steps to delete the first node and then insert a new node with value 25 at the beginning. What will be the final list?

hard

A. 10 -> 15 -> 20 -> 25 -> null

B. 25 -> 10 -> 15 -> 20 -> null

C. 5 -> 10 -> 15 -> 20 -> 25 -> null

D. 25 -> 5 -> 10 -> 15 -> 20 -> null

Solution

1. Step 1: Delete the first node

   Move head pointer from node 5 to node 10, so list becomes 10 -> 15 -> 20 -> null.

2. Step 2: Insert new node with value 25 at beginning

   Create new node with data 25, set its next to current head (node 10), then update head to new node.

3. Step 3: Final list after insertion

   List becomes 25 -> 10 -> 15 -> 20 -> null.

4. Final Answer:

   25 -> 10 -> 15 -> 20 -> null -> Option B

5. Quick Check:

   Delete first, then add new node at head [OK]

Hint: Delete head by moving to next, then add new node before head [OK]

Common Mistakes:

• Not updating head after deletion
• Appending new node at end instead of beginning
• Forgetting to link new node to old head