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.