Python Program to Implement Stack using Queue

In this Python program, we’ll be implementing a stack using two queues. The stack data structure follows the Last-In-First-Out (LIFO) principle, while queues follow the First-In-First-Out (FIFO) principle. By using two queues cleverly, we can mimic the behavior of a stack.

Problem Statement

Implement a stack data structure using two queues (queue1 and queue2) and provide methods to push, pop, and check if the stack is empty (is_empty).

Python Program to Implement Stack using Queue

class StackUsingQueue:
    def __init__(self):
        self.queue1 = []
        self.queue2 = []

    def push(self, value):
        self.queue1.append(value)

    def pop(self):
        if not self.queue1:
            return None

        while len(self.queue1) > 1:
            self.queue2.append(self.queue1.pop(0))

        popped_value = self.queue1.pop(0)

        self.queue1, self.queue2 = self.queue2, self.queue1

        return popped_value

    def is_empty(self):
        return len(self.queue1) == 0

# Create a stack instance
stack = StackUsingQueue()

# Push elements onto the stack
stack.push(10)
stack.push(20)
stack.push(30)

# Pop elements from the stack
print(stack.pop())  # Output: 30
print(stack.pop())  # Output: 20

# Check if the stack is empty
print(stack.is_empty())  # Output: False

print(stack.pop())  # Output: 10
print(stack.is_empty())  # Output: True

How It Works

• We use two queues, queue1 and queue2, to simulate the stack behavior.
• The push operation is straightforward and is performed on queue1.
• The pop operation is implemented by transferring all elements except the last one from queue1 to queue2. The last element is then popped from queue1, effectively mimicking the behavior of a stack’s pop operation.
• The is_empty method simply checks if queue1 is empty.

Input/Output