Implement Stack Using Queue in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue in the stack constructor.

DSA Python

import collections
class StackUsingQueue:
    def __init__(self):
        self.queue = [1]

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

Alist()

B[]

Ccollections.deque()

Dqueue.Queue()

Attempts:

3 left

💡 Hint

Common Mistakes

Using a plain list which is inefficient for queue operations.

Using queue.Queue() which is for threading and not needed here.

We use collections.deque() because it provides efficient queue operations.

2fill in blank

medium

Complete the code to add an element to the stack using the queue.

DSA Python

def push(self, x):
    self.queue.append(x)
    for _ in range(len(self.queue) - 1):
        self.queue.append([1])

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

Aself.queue.popitem()

Bself.queue.pop(0)

Cself.queue.pop()

Dself.queue.popleft()

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes from the end, not the front.

Using pop(0) on deque which is invalid.

We use popleft() to remove the front element and append it to the back, rotating the queue.

3fill in blank

hard

Fix the error in the pop method to remove and return the top element of the stack.

DSA Python

def pop(self):
    if not self.queue:
        return None
    return self.queue.[1]()

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

Apopleft

Bpop

Cremove

Ddequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using popleft() which removes from the front, not the top.

Using remove() which requires a value, not an index.

After rotation in push, the top element is at the end, so pop() removes it.

4fill in blank

hard

Fill both blanks to implement the top method that returns the top element without removing it.

DSA Python

def top(self):
    if not self.queue:
        return None
    return self.queue[1]

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

A[-1]

B[0]

C.pop()

D.popleft()

Attempts:

3 left

💡 Hint

Common Mistakes

Using [0] which accesses the front, not the top.

Using .pop() which removes the element.

The top element is at the end of the queue, so access it with [-1].

5fill in blank

hard

Fill all three blanks to implement the empty method that checks if the stack is empty.

DSA Python

def empty(self):
    return [1] == [2]

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

Alen(self.queue)

B0

C[]

Dself.queue

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing the queue object directly to an empty list.

Using self.queue == 0 which is invalid.

Check if the length of the queue is zero to determine if the stack is empty.

Practice

1. What is the main idea behind implementing a stack using a queue?

easy

A. Use a stack to implement queue operations

B. Use a queue to sort elements in ascending order

C. Use one or two queues to simulate stack operations like push and pop

D. Use two stacks to implement a queue

Solution

1. Step 1: Understand stack and queue differences

   A stack follows Last-In-First-Out (LIFO), while a queue follows First-In-First-Out (FIFO).

2. Step 2: Simulate stack using queue(s)

   By using one or two queues cleverly, we can reorder elements to behave like a stack for push and pop operations.

3. Final Answer:

   Use one or two queues to simulate stack operations like push and pop -> Option C

4. Quick Check:

   Stack via queue = simulate push/pop [OK]

Hint: Remember stack is LIFO, queue is FIFO; simulate LIFO with queue [OK]

Common Mistakes:

• Confusing stack and queue order
• Trying to sort queue instead of simulating stack
• Using stack to implement queue instead

2. Which of the following is the correct syntax to initialize an empty queue in Python using collections.deque?

easy

A. queue = deque()

B. queue = []

C. queue = Queue()

D. queue = queue()

Solution

1. Step 1: Recall Python queue initialization

   collections.deque() is the correct way to create a double-ended queue in Python.

2. Step 2: Check options

   Only queue = deque() uses deque() correctly; others are either list or undefined.

3. Final Answer:

   queue = deque() -> Option A

4. Quick Check:

   collections.deque() creates queue [OK]

Hint: Use deque() from collections for queue in Python [OK]

Common Mistakes:

• Using list [] instead of deque
• Calling undefined Queue() class
• Using lowercase queue() which is not defined

3. What will be the output of the following code implementing push and pop using one queue?

from collections import deque

class Stack:
    def __init__(self):
        self.q = deque()

    def push(self, x):
        self.q.append(x)
        for _ in range(len(self.q) - 1):
            self.q.append(self.q.popleft())

    def pop(self):
        return self.q.popleft() if self.q else None

s = Stack()
s.push(10)
s.push(20)
s.push(30)
print(s.pop())
print(s.pop())
print(s.pop())

medium

A. 20\n30\n10

B. 10\n20\n30

C. 30\n10\n20

D. 30\n20\n10

Solution

1. Step 1: Understand push operation

   Push appends element then rotates queue so new element is at front.

2. Step 2: Trace push and pop calls

   Push 10: queue=[10]
   Push 20: queue=[20,10]
   Push 30: queue=[30,20,10]
   Pop removes front: 30, then 20, then 10.

3. Final Answer:

   30 20 10 -> Option D

4. Quick Check:

   Stack pop order = 30,20,10 [OK]

Hint: Push rotates queue to front; pop removes front [OK]

Common Mistakes:

• Ignoring rotation in push
• Assuming pop removes last element instead of front
• Confusing queue order with stack order

4. Find the error in this stack implementation using two queues:

from collections import deque

class Stack:
    def __init__(self):
        self.q1 = deque()
        self.q2 = deque()

    def push(self, x):
        self.q1.append(x)

    def pop(self):
        while len(self.q1) > 1:
            self.q2.append(self.q1.popleft())
        return self.q1.popleft()
        self.q1, self.q2 = self.q2, self.q1

medium

A. The queues are not swapped after pop, so q1 becomes empty

B. The pop method does not remove the last element

C. The push method should append to q2 instead of q1

D. The pop method should return from q2 instead of q1

Solution

1. Step 1: Analyze pop method flow

   Pop moves all but last from q1 to q2, then pops last from q1.

2. Step 2: Identify missing swap

   After popping, q1 is empty but queues are not swapped, so next push/pop fail.

3. Final Answer:

   The queues are not swapped after pop, so q1 becomes empty -> Option A

4. Quick Check:

   Missing swap causes empty q1 [OK]

Hint: Swap queues after pop to keep q1 as main queue [OK]

Common Mistakes:

• Not swapping queues after pop
• Returning before swapping queues
• Appending to wrong queue in push

5. You want to implement a stack using a single queue with minimal operations. Which approach correctly achieves this for the push operation to maintain stack order?

hard

A. Use two queues to enqueue the new element in the empty queue

B. Append the new element, then rotate the queue by dequeuing and enqueuing all previous elements

C. Dequeue all elements, enqueue the new element, then enqueue all dequeued elements

D. Append the new element and do nothing else

Solution

1. Step 1: Understand stack order with one queue

   To simulate LIFO, after adding new element, rotate queue so new element is front.

2. Step 2: Compare approaches

   Append the new element, then rotate the queue by dequeuing and enqueuing all previous elements rotates queue after append, maintaining stack order. Append the new element and do nothing else does not reorder. Dequeue all elements, enqueue the new element, then enqueue all dequeued elements is inefficient but correct logically. Use two queues to enqueue the new element in the empty queue uses two queues, not single.

3. Final Answer:

   Append the new element, then rotate the queue by dequeuing and enqueuing all previous elements -> Option B

4. Quick Check:

   Rotate after push keeps stack order [OK]

Hint: Rotate queue after push to keep new element on top [OK]

Common Mistakes:

• Not rotating queue after push
• Using two queues when only one is allowed
• Appending without reordering elements