Reverse a Doubly Linked List in DSA Python - Build from Scratch

Reverse a Doubly Linked List

📖 Scenario: You are working on a music playlist app. The playlist is stored as a doubly linked list where each node contains a song name. You want to add a feature to reverse the order of songs in the playlist.

🎯 Goal: Build a program that creates a doubly linked list of songs, then reverses the list, and finally prints the reversed playlist.

📋 What You'll Learn

Create a doubly linked list with exactly these songs in order: 'Song1', 'Song2', 'Song3', 'Song4'

Create a variable called head that points to the first node of the doubly linked list

Write a function called reverse_doubly_linked_list(head) that reverses the doubly linked list and returns the new head

Print the reversed doubly linked list in the format: Song4 <-> Song3 <-> Song2 <-> Song1 <-> None

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps where you need to move forward and backward through items efficiently.

💼 Career

Understanding linked lists and how to reverse them is a common interview question and helps build strong problem-solving skills in data structures.

1

Create the doubly linked list with songs

Create a class called Node with attributes data, prev, and next. Then create four nodes with data 'Song1', 'Song2', 'Song3', and 'Song4'. Link them to form a doubly linked list. Finally, create a variable called head that points to the first node.

DSA Python

# Define Node class and create doubly linked list nodes
# Your code here

Hint

Remember to link each node's next and prev attributes to form the doubly linked list.

2

Create a function to reverse the doubly linked list

Define a function called reverse_doubly_linked_list(head) that takes the head of the doubly linked list and reverses the list by swapping the next and prev pointers of each node. The function should return the new head of the reversed list.

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')
node4 = Node('Song4')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2
node3.next = node4
node4.prev = node3

head = node1

# Define reverse_doubly_linked_list function below
# Your code here

Hint

Swap the prev and next pointers for each node. Move to the original next node by using the updated prev pointer.

3

Reverse the doubly linked list using the function

Call the function reverse_doubly_linked_list(head) and assign its result back to the variable head to update the head to the new reversed list.

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')
node4 = Node('Song4')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2
node3.next = node4
node4.prev = node3

head = node1

def reverse_doubly_linked_list(head):
    current = head
    temp = None
    while current is not None:
        temp = current.prev
        current.prev = current.next
        current.next = temp
        current = current.prev
    if temp is not None:
        head = temp.prev
    return head

# Call reverse_doubly_linked_list and update head
# Your code here

Hint

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

4

Print the reversed doubly linked list

Write a loop to print the reversed doubly linked list starting from head. Print each node's data followed by ' <-> ' and end with 'None'. The output should look like: Song4 <-> Song3 <-> Song2 <-> Song1 <-> 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')
node4 = Node('Song4')

node1.next = node2
node2.prev = node1
node2.next = node3
node3.prev = node2
node3.next = node4
node4.prev = node3

head = node1

def reverse_doubly_linked_list(head):
    current = head
    temp = None
    while current is not None:
        temp = current.prev
        current.prev = current.next
        current.next = temp
        current = current.prev
    if temp is not None:
        head = temp.prev
    return head

head = reverse_doubly_linked_list(head)

# Print the reversed doubly linked list
# Your code here

Hint

Use a while loop to traverse from head and print each node's data followed by ' <-> '. End with printing 'None'.