Implement Stack Using Queue in DSA Python - Build from Scratch

Implement Stack Using Queue

📖 Scenario: Imagine you have a stack of books but you only have a queue (a line) to hold them. You want to use the queue to behave like a stack, where you can add and remove books from the top only.

🎯 Goal: You will build a stack using a queue. You will add items to the stack and remove the top item using only queue operations.

📋 What You'll Learn

Create a queue using Python's collections.deque

Implement push method to add an item to the stack

Implement pop method to remove the top item from the stack

Use only queue operations to simulate stack behavior

💡 Why This Matters

🌍 Real World

Sometimes hardware or software only provides queue operations, but you need stack behavior. This technique helps in such cases.

💼 Career

Understanding how to simulate one data structure using another is useful for coding interviews and system design.

1

Create a queue using deque

Import deque from collections and create a queue called queue using deque().

DSA Python

# Import deque and create queue
# Your code here

Hint

Use from collections import deque and then queue = deque().

2

Add a push function to add items to the stack

Define a function called push that takes a parameter item. Inside it, add item to the right end of queue using queue.append(item).

DSA Python

from collections import deque
queue = deque()

# Define push function to add item to queue
# Your code here

Hint

Use def push(item): and inside it call queue.append(item).

3

Add a pop function to remove the top item from the stack

Define a function called pop that removes and returns the last added item from queue. Use a loop to move all items except the last from the front to the back of queue. Then remove and return the last item using queue.popleft().

DSA Python

from collections import deque
queue = deque()

def push(item):
    queue.append(item)

# Define pop function to remove and return top item
# Your code here

Hint

Use a for loop to move items from front to back except the last, then remove the last with queue.popleft().

4

Test the stack by pushing and popping items

Call push(10), push(20), then print the result of pop() and pop() to show the stack behavior.

DSA Python

from collections import deque
queue = deque()

def push(item):
    queue.append(item)

def pop():
    for _ in range(len(queue) - 1):
        queue.append(queue.popleft())
    return queue.popleft()

# Push 10 and 20, then print pop results
# Your code here

Hint

Push 10 then 20, then pop twice. The output should be 20 then 10.

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