Queue Implementation Using Linked List in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a node with given data.

DSA Python

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

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

Aself

Bnext

CNode

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'data' for the node's data.

Assigning 'self' or 'Node' instead of the parameter.

The node stores the given data in self.data. So, we assign the parameter data to self.data.

2fill in blank

medium

Complete the enqueue method to add a new node at the end of the queue.

DSA Python

def enqueue(self, data):
    new_node = Node(data)
    if self.rear is None:
        self.front = self.rear = new_node
    else:
        self.rear.next = [1]
        self.rear = new_node

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

Anew_node

Bself.front

Cself.rear

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning rear.next to self.front or self.rear instead of new_node.

Assigning rear.next to data instead of a node.

To add a new node at the end, set the current rear's next pointer to the new node.

3fill in blank

hard

Fix the error in the dequeue method to correctly remove the front node.

DSA Python

def dequeue(self):
    if self.front is None:
        return None
    temp = self.front
    self.front = self.front.[1]
    if self.front is None:
        self.rear = None
    return temp.data

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

Aprev

Bnext

Cfront

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which does not exist in singly linked list nodes.

Using 'data' or 'front' which are not pointers to the next node.

To remove the front node, update self.front to point to the next node after the current front.

4fill in blank

hard

Fill both blanks to check if the queue is empty.

DSA Python

def is_empty(self):
    return self.[1] is [2]

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

Afront

Brear

CNone

DTrue

Attempts:

3 left

💡 Hint

Common Mistakes

Checking rear instead of front.

Comparing to True instead of None.

The queue is empty if the front pointer is None.

5fill in blank

hard

Fill all three blanks to implement a method that prints all elements in the queue.

DSA Python

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

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

Afront

BNone

Cdata

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from rear instead of front.

Checking for True instead of None in the loop condition.

Printing 'next' instead of 'data'.

Start from the front node, loop until current is None, and print each node's data.

Practice

1. What is the main advantage of implementing a queue using a linked list instead of an array?

easy

A. Simpler to implement than arrays

B. Faster access to middle elements

C. Uses less memory for small queues

D. Dynamic size without fixed capacity limit

Solution

1. Step 1: Understand queue size limitation in arrays

   Arrays have fixed size, so queues implemented with arrays have a maximum capacity.

2. Step 2: Linked list dynamic sizing

   Linked lists can grow or shrink dynamically as nodes are added or removed, so no fixed size limit.

3. Final Answer:

   Dynamic size without fixed capacity limit -> Option D

4. Quick Check:

   Linked list queues grow dynamically = A [OK]

Hint: Linked lists grow dynamically, arrays have fixed size [OK]

Common Mistakes:

• Thinking linked lists allow faster middle access
• Assuming linked lists always use less memory
• Confusing implementation simplicity

2. Which of the following is the correct way to initialize an empty queue using a linked list in Python?

easy

A. queue = LinkedList()

B. queue = None

C. queue = Queue() with head and tail as None

D. queue = []

Solution

1. Step 1: Understand queue initialization

   An empty queue using linked list usually has head and tail pointers set to None.

2. Step 2: Identify correct initialization

   queue = Queue() with head and tail as None correctly describes a Queue class with head and tail as None, representing empty queue.

3. Final Answer:

   queue = Queue() with head and tail as None -> Option C

4. Quick Check:

   Empty queue has head and tail None = B [OK]

Hint: Empty linked list queue has head and tail as None [OK]

Common Mistakes:

• Using list [] instead of linked list
• Setting queue to None without structure
• Confusing linked list with array initialization

3. What will be the output after performing these operations on an empty linked list queue?

enqueue(10)
enqueue(20)
dequeue()
enqueue(30)
dequeue()

Print the queue elements from front to rear.

medium

A. 30 -> null

B. 20 -> 30 -> null

C. 10 -> 20 -> 30 -> null

D. 20 -> null

Solution

1. Step 1: Perform enqueue(10) and enqueue(20)

   Queue becomes 10 -> 20 -> null.

2. Step 2: Perform dequeue()

   Removes 10, queue is now 20 -> null.

3. Step 3: Perform enqueue(30)

   Queue becomes 20 -> 30 -> null.

4. Step 4: Perform dequeue()

   Removes 20, queue is now 30 -> null.

5. Final Answer:

   30 -> null -> Option A

6. Quick Check:

   After operations, queue front is 30 [OK]

Hint: Track enqueue/dequeue stepwise to find final queue [OK]

Common Mistakes:

• Forgetting to remove front element on dequeue
• Appending dequeue element instead of removing
• Mixing order of operations

4. Identify the error in this dequeue method of a linked list queue:

def dequeue(self):
    if self.head is None:
        return None
    temp = self.head
    self.head = self.head.next
    return temp.data
    self.tail = None

medium

A. The tail is set to None after return, so never updated

B. The method does not check if queue is empty

C. It should update head before returning data

D. temp.data is not a valid attribute

Solution

1. Step 1: Analyze the order of statements

   The return statement occurs before setting tail to None, so tail update never happens.

2. Step 2: Understand impact on queue state

   If last element is dequeued, tail should be set to None to mark empty queue, but it is skipped.

3. Final Answer:

   The tail is set to None after return, so never updated -> Option A

4. Quick Check:

   Return before tail update skips tail reset [OK]

Hint: Return ends function; update tail before return [OK]

Common Mistakes:

• Placing return before all updates
• Not resetting tail when queue becomes empty
• Ignoring empty queue check

5. You want to implement a method to reverse the queue implemented using a linked list without using extra data structures. Which approach is best?

hard

A. Create a new linked list and enqueue elements from old queue in reverse

B. Use recursion to dequeue and enqueue elements in reverse order

C. Traverse the linked list and swap data of nodes pairwise

D. Use a stack to store elements and then enqueue back

Solution

1. Step 1: Understand constraints

   No extra data structures allowed, so stack or new list are not options.

2. Step 2: Use recursion to reverse queue

   Recursion uses call stack implicitly to dequeue all elements, then enqueue them back in reverse order.

3. Final Answer:

   Use recursion to dequeue and enqueue elements in reverse order -> Option B

4. Quick Check:

   Recursion reverses queue without extra structures [OK]

Hint: Recursion uses call stack to reverse without extra structures [OK]

Common Mistakes:

• Using extra data structures despite constraints
• Swapping data pairs does not reverse queue order
• Creating new linked list violates no extra structure rule