stacks_using_queues.py

'''
Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.
Notes:

You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid. 

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

'''
from collections import deque

class MyStack:
    def __init__(self):
        self.queue1 = deque()
        self.queue2 = deque()
        
    def push(self, x: int) -> None:
        self.queue1.append(x)

    def pop(self) -> int:
        #move all elements except the last from queue1 to queue2
        while len(self.queue1) > 1:
            self.queue2.append(self.queue1.popleft())
        
        #the last element in queue1 is in top of the stack
        top_element = self.queue1.popleft()

        #swap the name of the queues
        self.queue1, self.queue2 = self.queue2, self.queue1

        return top_element

    def top(self) -> int:
        #move all elements except the last from queue1 to queue2
        while len(self.queue1) > 1:
            self.queue2.append(self.queue1.popleft())

        #the last element in queue1 is in top of the stack
        top_element = self.queue1.popleft()
        self.queue2.append(top_element)

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

        return top_element
        

    def empty(self) -> bool:
        return not self.queue1 and not self.queue2


obj = MyStack()
obj.push(1)
obj.push(2)
print(obj.pop())
print(obj.top())
print(obj.empty())