Delete from Beginning of Doubly Linked List in DSA Python - Build from Scratch

Delete from Beginning of Doubly Linked List

📖 Scenario: You are managing a playlist of songs where each song is linked to the previous and next song. Sometimes, you want to remove the first song from the playlist.

🎯 Goal: Build a doubly linked list with three songs, then delete the first song from the list, and finally print the updated playlist.

📋 What You'll Learn

Create a doubly linked list with three nodes containing the exact song names: 'Song1', 'Song2', 'Song3'.

Create a variable called head pointing to the first node.

Write a function called delete_from_beginning that removes the first node from the doubly linked list.

Print the updated list starting from head after deletion.

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, web browsers (back and forward navigation), and undo-redo features where you need to move both forward and backward.

💼 Career

Understanding how to manipulate doubly linked lists is important for software engineers working on systems that require efficient insertions and deletions from both ends, such as caches and navigation history.

1

Create the Doubly Linked List with Three Songs

Create a class called Node with attributes data, prev, and next. Then create three nodes with data 'Song1', 'Song2', and 'Song3'. Link them to form a doubly linked list and set head to the first node.

DSA Python

# Define Node class and create three linked nodes
# Your code here

Hint

Remember to link nodes both ways: next and prev.

2

Create a Function to Delete from Beginning

Create a function called delete_from_beginning that takes head as input and returns the new head after deleting the first node. If the list is empty, return None. If there is a next node, update its prev to None.

DSA Python

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

node1 = Node('Song1')
node2 = Node('Song2')
node3 = Node('Song3')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2

head = node1

# Define delete_from_beginning function below
# Your code here

Hint

Check if head is None first. Then update new_head.prev to None if it exists.

3

Delete the First Node from the List

Call the function delete_from_beginning with the variable head and update head with the returned new head.

DSA Python

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

node1 = Node('Song1')
node2 = Node('Song2')
node3 = Node('Song3')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2

head = node1

def delete_from_beginning(head):
    if head is None:
        return None
    new_head = head.next
    if new_head is not None:
        new_head.prev = None
    return new_head

# Call delete_from_beginning and update head
# Your code here

Hint

Assign the result of delete_from_beginning(head) back to head.

4

Print the Updated Doubly Linked List

Use a while loop with a variable current starting at head to print the data of each node separated by -> . End the print with null.

DSA Python

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

node1 = Node('Song1')
node2 = Node('Song2')
node3 = Node('Song3')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2

head = node1

def delete_from_beginning(head):
    if head is None:
        return None
    new_head = head.next
    if new_head is not None:
        new_head.prev = None
    return new_head

head = delete_from_beginning(head)

# Print the doubly linked list starting from head
# Your code here

Hint

Use a loop to print each node's data followed by -> , then print null at the end.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It moves to the second node in the list

B. It becomes null immediately

C. It points to the last node in the list

D. It remains pointing to the deleted node

Solution

1. Step 1: Understand the head pointer role

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

2. Step 2: After deleting the first node

   The head must move to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option A

4. Quick Check:

   Head points to new first node after deletion [OK]

Hint: Head always points to the first node, update after deletion [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null even if list has more nodes
• Leaving head pointing to deleted node

2. Which of the following is the correct syntax to delete the first node in a doubly linked list in Python?

easy

A. head = None; head.next = None

B. head.prev = None; head = head.next

C. head = head.next; head.prev = None

D. head.next = None; head = head.prev

Solution

1. Step 1: Move head to next node

   Assign head = head.next to move head forward.

2. Step 2: Remove backward link

   Set head.prev = None to disconnect previous first node.

3. Final Answer:

   head = head.next; head.prev = None -> Option C

4. Quick Check:

   Update head then clear prev link [OK]

Hint: Update head first, then clear prev pointer [OK]

Common Mistakes:

• Setting prev before moving head
• Trying to set next of head to None incorrectly
• Setting head to None without updating

3. What will be the output linked list after deleting the first node from this doubly linked list?

Initial list: 10 <-> 20 <-> 30 <-> null

Code snippet:

head = head.next
head.prev = None

medium

A. 20 <-> 30 <-> null

B. 10 <-> 30 <-> null

C. 30 <-> null

D. null

Solution

1. Step 1: Move head to second node

   Head moves from 10 to 20.

2. Step 2: Remove backward link of new head

   Set 20.prev = None, disconnecting 10.

3. Final Answer:

   20 <-> 30 <-> null -> Option A

4. Quick Check:

   First node removed, list starts at 20 [OK]

Hint: After deletion, head points to second node [OK]

Common Mistakes:

• Forgetting to set prev to None
• Incorrectly skipping nodes
• Outputting original list without change

4. Identify the error in this code snippet for deleting the first node of a doubly linked list:

if head is not None:
    head = head.next
    head.prev = None

medium

A. No error, code is correct

B. head.prev = None causes error if head.next is None

C. head = head.next should be after setting prev to None

D. Missing deletion of the old head node

Solution

1. Step 1: Check head.next for None

   If head.next is None, head becomes None after assignment.

2. Step 2: Setting head.prev causes error

   Trying to set head.prev = None when head is None causes AttributeError.

3. Final Answer:

   head.prev = None causes error if head.next is None -> Option B

4. Quick Check:

   Check for None before setting prev [OK]

Hint: Check if head is None before setting head.prev [OK]

Common Mistakes:

• Not checking if head.next is None
• Assuming list has more than one node always
• Ignoring possible AttributeError

5. Given a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> null, after deleting the first node twice, what will be the resulting list?

hard

A. null

B. 15 <-> 25 <-> 35 <-> null

C. 35 <-> null

D. 25 <-> 35 <-> null

Solution

1. Step 1: First deletion moves head from 5 to 15

   After first deletion, list is 15 <-> 25 <-> 35 <-> null.

2. Step 2: Second deletion moves head from 15 to 25

   After second deletion, list is 25 <-> 35 <-> null.

3. Final Answer:

   25 <-> 35 <-> null -> Option D

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head to next node [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head correctly after each deletion
• Confusing node values or order