Delete Node at Beginning in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after deleting the first node?

Consider a singly linked list with nodes 10 -> 20 -> 30 -> null. After deleting the node at the beginning, what is the printed state of the list?

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 self.head:
            self.head = self.head.next

    def print_list(self):
        current = self.head
        while current:
            print(current.data, end=' -> ')
            current = current.next
        print('null')

ll = LinkedList()
ll.append(10)
ll.append(20)
ll.append(30)
ll.delete_at_beginning()
ll.print_list()

A null

B 10 -> 20 -> 30 -> null

C 20 -> 30 -> null

D 30 -> null

💡 Hint

Deleting at beginning removes the first node and moves head to next node.

Initially, the list is 10 -> 20 -> 30 -> null. Deleting the first node removes 10, so the new head is 20, resulting in 20 -> 30 -> null.

❓ Predict Output

intermediate

What happens when deleting from a single-node list?

Given a linked list with a single node 5 -> null, what is the printed output after deleting the node at the beginning?

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 self.head:
            self.head = self.head.next

    def print_list(self):
        current = self.head
        while current:
            print(current.data, end=' -> ')
            current = current.next
        print('null')

ll = LinkedList()
ll.append(5)
ll.delete_at_beginning()
ll.print_list()

A 5 -> null

B null

C Error

D 5

💡 Hint

Deleting the only node leaves the list empty.

After deleting the only node, the head becomes None, so printing the list shows 'null'.

🔧 Debug

advanced

Identify the error in this delete at beginning method

What error will this code produce when called on an empty list?

DSA Python

def delete_at_beginning(self):
    if self.head.next:
        self.head = self.head.next
    else:
        self.head = None

A AttributeError when list is empty

B No error, works correctly

C TypeError when list is empty

D IndexError when list has multiple nodes

💡 Hint

Check what happens if head is None before accessing head.next.

If the list is empty (head is None), accessing head.next causes AttributeError. The code does not check if head is None before accessing head.next.

🧠 Conceptual

advanced

What is the time complexity of deleting the first node in a singly linked list?

Consider a singly linked list with n nodes. What is the time complexity of deleting the node at the beginning?

A O(1)

B O(n)

C O(log n)

D O(n^2)

💡 Hint

Deleting the first node only changes the head pointer.

Deleting the first node requires only updating the head pointer to the next node, which is a constant time operation.

🚀 Application

expert

After multiple deletions at beginning, what is the list state?

Starting with the list 1 -> 2 -> 3 -> 4 -> 5 -> null, after deleting the first node three times, what is the printed list?

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 self.head:
            self.head = self.head.next

    def print_list(self):
        current = self.head
        while current:
            print(current.data, end=' -> ')
            current = current.next
        print('null')

ll = LinkedList()
for i in range(1, 6):
    ll.append(i)
ll.delete_at_beginning()
ll.delete_at_beginning()
ll.delete_at_beginning()
ll.print_list()

A 2 -> 3 -> 4 -> 5 -> null

B 3 -> 4 -> 5 -> null

C null

D 4 -> 5 -> null

💡 Hint

Each deletion removes the current head node.

Deleting three times removes nodes 1, 2, and 3. The new head is node 4, so the list is 4 -> 5 -> null.

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