Queue Implementation Using Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Enqueue and Dequeue Operations

What is the printed state of the queue after performing these operations?

Operations:
1. Enqueue 10
2. Enqueue 20
3. Enqueue 30
4. Dequeue
5. Enqueue 40

DSA Python

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

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None

    def enqueue(self, data):
        new_node = Node(data)
        if self.rear is None:
            self.front = self.rear = new_node
            return
        self.rear.next = new_node
        self.rear = new_node

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

    def print_queue(self):
        current = self.front
        result = []
        while current:
            result.append(current.data)
            current = current.next
        print(" -> ".join(map(str, result)) + " -> null")

q = Queue()
q.enqueue(10)
q.enqueue(20)
q.enqueue(30)
q.dequeue()
q.enqueue(40)
q.print_queue()

A 10 -> 20 -> 30 -> 40 -> null

B 20 -> 30 -> 40 -> null

C 30 -> 40 -> null

D 40 -> null

💡 Hint

Remember that dequeue removes from the front of the queue.

After enqueueing 10, 20, 30, the queue is 10 -> 20 -> 30 -> null. Dequeue removes 10, so queue becomes 20 -> 30 -> null. Enqueue 40 adds it at the rear, so final queue is 20 -> 30 -> 40 -> null.

🧠 Conceptual

intermediate

Queue Front and Rear After Operations

After performing these operations on an empty queue:
enqueue(5), enqueue(15), dequeue(), enqueue(25), dequeue()
What are the values at the front and rear of the queue?

A Front: 5, Rear: 25

B Front: 15, Rear: 25

C Front: 25, Rear: 15

D Front: 25, Rear: 25

💡 Hint

Track each operation carefully and update front and rear pointers.

Operations step-by-step: - enqueue(5): front=5, rear=5 - enqueue(15): front=5, rear=15 - dequeue(): removes 5, front=15, rear=15 - enqueue(25): front=15, rear=25 - dequeue(): removes 15, front=25, rear=25 So both front and rear point to 25.

🔧 Debug

advanced

Identify the Bug in Dequeue Method

What error will this code raise when dequeue is called on an empty queue?

DSA Python

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

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None

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

q = Queue()
q.dequeue()

A AttributeError: 'NoneType' object has no attribute 'next'

B IndexError: dequeue from empty queue

C TypeError: unsupported operand type(s)

D No error, returns null

💡 Hint

Check what happens when front is null and you try to access temp.next.

When the queue is empty, front is null. The code tries to access temp.next where temp is null, causing AttributeError.

📝 Syntax

advanced

Syntax Error in Enqueue Method

Which option contains the correct enqueue method for a linked list queue?

A

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

B

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

C

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

D

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

💡 Hint

Look for missing colons and proper if-else structure.

Option A has correct syntax with colons after if and else, and proper assignment. Options B and C miss colons causing SyntaxError. Option A has an unnecessary return but is syntactically valid; however, the trailing comma in the option text is not part of code and does not cause error, so option A is valid syntax but the trailing comma is not code. To avoid ambiguity, option A is the best correct answer.

🚀 Application

expert

Queue Size After Mixed Operations

Given the following sequence of operations on an empty queue, what is the size of the queue at the end?

enqueue(1), enqueue(2), dequeue(), enqueue(3), enqueue(4), dequeue(), dequeue(), enqueue(5)

DSA Python

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

class Queue:
    def __init__(self):
        self.front = None
        self.rear = None
        self.size = 0

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

    def dequeue(self):
        if self.front is None:
            return None
        temp = self.front
        self.front = temp.next
        if self.front is None:
            self.rear = None
        self.size -= 1
        return temp.data

q = Queue()
q.enqueue(1)
q.enqueue(2)
q.dequeue()
q.enqueue(3)
q.enqueue(4)
q.dequeue()
q.dequeue()
q.enqueue(5)
print(q.size)

A 1

B 3

C 2

D 4

💡 Hint

Count each enqueue as +1 and each dequeue as -1 carefully.

Operations and size changes: - enqueue(1): size=1 - enqueue(2): size=2 - dequeue(): size=1 - enqueue(3): size=2 - enqueue(4): size=3 - dequeue(): size=2 - dequeue(): size=1 - enqueue(5): size=2 Final size is 2.