stack_using_two_queues.py

from collections import deque

class StackUsingQueues:

    def __init__(self):
        self.queue1 = deque()
        self.queue2 = deque()

    def push(self, value: int) -> None:
        """
        Pushes an element onto the stack.

        Args:
            value (int): The element to push onto the stack.
        
        Example:
            >>> stack = StackUsingQueues()
            >>> stack.push(1)
            >>> stack.push(2)
            >>> stack.push(3)
        """
        self.queue2.append(value)
        while self.queue1:
            self.queue2.append(self.queue1.popleft())
        self.queue1, self.queue2 = self.queue2, self.queue1

    def pop(self) -> None:
        """
        Pops and removes the top element from the stack.

        Example:
            >>> stack = StackUsingQueues()
            >>> stack.push(1)
            >>> stack.push(2)
            >>> stack.pop()
        """
        if self.queue1:
            self.queue1.popleft()

    def top(self) -> int:
        """
        Returns the top element of the stack without removing it.

        Returns:
            int: The top element of the stack.
        
        Example:
            >>> stack = StackUsingQueues()
            >>> stack.push(1)
            >>> stack.push(2)
            >>> stack.top()
            2
        """
        if self.queue1:
            return self.queue1[0]
        return None

    def size(self) -> int:
        """
        Returns the current size (number of elements) of the stack.

        Returns:
            int: The size of the stack.
        
        Example:
            >>> stack = StackUsingQueues()
            >>> stack.size()
            0
            >>> stack.push(1)
            >>> stack.push(2)
            >>> stack.push(3)
            >>> stack.size()
            3
            >>> stack.pop()
            >>> stack.size()
            2
        """
        return len(self.queue1)

    def is_empty(self) -> bool:
        """
        Checks if the stack is empty.

        Returns:
            bool: True if the stack is empty, False otherwise.
        
        Example:
            >>> stack = StackUsingQueues()
            >>> stack.is_empty()
            True
            >>> stack.push(1)
            >>> stack.is_empty()
            False
        """
        return len(self.queue1) == 0

    def peek(self) -> int:
        """
        Returns the top element of the stack without removing it.

        Returns:
            int: The top element of the stack.
        
        Example:
            >>> stack = StackUsingQueues()
            >>> stack.push(1)
            >>> stack.push(2)
            >>> stack.peek()
            2
        """
        if self.queue1:
            return self.queue1[0]
        return None

if __name__ == "__main__":
    stack = StackUsingQueues()

    # Push some elements onto the stack
    stack.push(1)
    stack.push(2)
    stack.push(3)

    print("Stack size:", stack.size())
    print("Top element:", stack.top())

    # Pop elements from the stack
    stack.pop()
    print("Top element after pop:", stack.top())

    stack.pop()
    print("Top element after another pop:", stack.top())

    print("Is stack empty?", stack.is_empty())
    print("Peek at the top element:", stack.peek())
    print("Final stack size:", stack.size())