Doubly Linked List Structure and Node Design in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Node class with a pointer to the next node.

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.next = [1]
        self.prev = None

Drag options to blanks, or click blank then click option'

ANone

Bself

C0

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Setting next to 0 or data instead of None.

Confusing next with prev pointer.

The next pointer should be initialized to None because the node does not point to any other node yet.

2fill in blank

medium

Complete the code to initialize the previous pointer in the Node class.

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = [1]

Drag options to blanks, or click blank then click option'

A0

Bself

Cdata

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Setting prev to 0 or data instead of None.

Confusing prev with next pointer.

The prev pointer should be None initially because the node has no previous node yet.

3fill in blank

hard

Fix the error in the DoublyLinkedList class constructor to initialize the head pointer.

DSA Python

class DoublyLinkedList:
    def __init__(self):
        self.head = [1]

Drag options to blanks, or click blank then click option'

A0

BNone

Cself

DNode

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to 0 or Node instead of None.

Assigning head to self which is incorrect.

The head pointer should be None initially because the list is empty.

4fill in blank

hard

Fill both blanks to complete the method that adds a new node at the beginning of the doubly linked list.

DSA Python

def add_at_beginning(self, data):
    new_node = Node(data)
    new_node.next = [1]
    if self.head is not None:
        self.head.prev = [2]
    self.head = new_node

Drag options to blanks, or click blank then click option'

Aself.head

Bnew_node

Cdata

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping new_node and self.head in assignments.

Forgetting to update the prev pointer of the old head.

The new node's next points to the current head, and the current head's prev points back to the new node.

5fill in blank

hard

Fill all three blanks to complete the method that prints the doubly linked list from head to tail.

DSA Python

def print_list(self):
    current = self.head
    while current is not [1]:
        print(current.[2], end=' <-> ')
        current = current.[3]
    print('None')

Drag options to blanks, or click blank then click option'

ANone

Bdata

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using prev instead of next to move forward.

Checking for current == 0 instead of None.

We loop until current is None, print the data, and move to the next node.

Practice

1. What is the main difference between a doubly linked list node and a singly linked list node?

easy

A. A doubly linked list node does not store any data.

B. A doubly linked list node has pointers to both the previous and next nodes.

C. A doubly linked list node can only point to the next node.

D. A doubly linked list node stores data in a different format.

Solution

1. Step 1: Understand node pointers in singly linked list

   A singly linked list node has only one pointer to the next node.

2. Step 2: Understand node pointers in doubly linked list

   A doubly linked list node has two pointers: one to the previous node and one to the next node.

3. Final Answer:

   A doubly linked list node has pointers to both the previous and next nodes. -> Option B

4. Quick Check:

   Doubly linked list nodes have two pointers = D [OK]

Hint: Remember: doubly means two pointers, singly means one [OK]

Common Mistakes:

• Thinking doubly linked list nodes store data differently
• Confusing pointer count between singly and doubly linked lists
• Assuming doubly linked list nodes have only one pointer

2. Which of the following is the correct way to define a doubly linked list node class in Python?

easy

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

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

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

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

Solution

1. Step 1: Check for data storage and pointers

   A doubly linked list node must store data and have both prev and next pointers.

2. Step 2: Verify class definitions

   class Node: def __init__(self, data): self.data = data self.prev = None self.next = None correctly defines data, prev, and next attributes in __init__.

3. Final Answer:

   class Node:\n def __init__(self, data):\n self.data = data\n self.prev = None\n self.next = None -> Option D

4. Quick Check:

   Node class with data, prev, next = B [OK]

Hint: Doubly linked nodes need both prev and next pointers [OK]

Common Mistakes:

• Omitting the prev pointer
• Not initializing data attribute
• Defining only next pointer like singly linked list

3. What will be the output of the following code snippet?

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

head = Node(1)
second = Node(2)
third = Node(3)

head.next = second
second.prev = head
second.next = third
third.prev = second

current = third
result = []
while current:
    result.append(current.data)
    current = current.prev
print(result)

medium

A. [3, 2, 1]

B. [1, 2, 3]

C. [1, 3, 2]

D. [3, 1, 2]

Solution

1. Step 1: Understand the linked list connections

   Nodes are linked as 1 <-> 2 <-> 3 with prev and next pointers set correctly.

2. Step 2: Trace the while loop starting from third node

   Starting at node with data 3, append 3, move to prev (2), append 2, move to prev (1), append 1, then prev is None, loop ends.

3. Final Answer:

   [3, 2, 1] -> Option A

4. Quick Check:

   Traverse backward from third node = [3, 2, 1] [OK]

Hint: Backward traversal collects nodes in reverse order [OK]

Common Mistakes:

• Confusing next and prev pointers during traversal
• Starting traversal from head instead of third
• Appending data in wrong order

4. Identify the error in the following doubly linked list node insertion code and select the correct fix.

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

head = Node(1)
new_node = Node(2)

# Insert new_node after head
new_node.next = head.next
head.next.prev = new_node
head.next = new_node
new_node.prev = head

medium

A. No error; code inserts node correctly.

B. new_node.prev should be assigned before head.next.

C. head.next.prev causes AttributeError; fix by checking if head.next is not None before accessing prev.

D. new_node.next should be assigned after head.next.

Solution

1. Step 1: Analyze head.next before insertion

   Initially, head.next is None, so head.next.prev causes AttributeError.

2. Step 2: Fix by checking if head.next exists before accessing prev

   Adding a condition to check if head.next is not None before setting head.next.prev avoids error.

3. Final Answer:

   head.next.prev causes AttributeError; fix by checking if head.next is not None before accessing prev. -> Option C

4. Quick Check:

   Check for None before accessing prev pointer [OK]

Hint: Always check if next node exists before accessing its prev [OK]

Common Mistakes:

• Ignoring None check causing runtime error
• Assigning pointers in wrong order
• Assuming head.next is always valid

5. You want to insert a new node with value 4 between nodes with values 2 and 3 in a doubly linked list. Given the nodes are connected as 1 <-> 2 <-> 3, which sequence of pointer updates correctly inserts the new node?

hard

A. new_node.prev = node2; new_node.next = node3; node2.next = new_node; node3.prev = new_node

B. new_node.next = node2; new_node.prev = node3; node3.next = new_node; node2.prev = new_node

C. node2.next = new_node; new_node.next = node3; node3.next = new_node; new_node.prev = node2

D. node3.prev = new_node; new_node.prev = node2; node2.next = new_node; new_node.next = node2

Solution

1. Step 1: Set new_node pointers to neighbors

   new_node.prev should point to node2 and new_node.next to node3 to link correctly.

2. Step 2: Update neighbors to point to new_node

   node2.next must point to new_node and node3.prev must point to new_node to complete insertion.

3. Final Answer:

   new_node.prev = node2; new_node.next = node3; node2.next = new_node; node3.prev = new_node -> Option A

4. Quick Check:

   Set new_node pointers then update neighbors [OK]

Hint: Link new node to neighbors, then neighbors back to new node [OK]

Common Mistakes:

• Swapping prev and next pointers
• Updating neighbors before setting new_node pointers
• Forgetting to update one neighbor pointer