[E].[Py].232.ImplementQueueUsingStacks

class MyQueue:

    def __init__(self):
        self.__s1 = Stack()
        self.__s2 = Stack()

    def push(self, x: int) -> None:
        self.__s1.push(x)

    def pop(self) -> int:
        while self.__s1.size() > 1:
            self.__s2.push(self.__s1.pop())
        returnValue = self.__s1.pop()
        while self.__s2.size() > 0:
            self.__s1.push(self.__s2.pop())
        return returnValue

    def peek(self) -> int:
        while self.__s1.size() > 1:
            self.__s2.push(self.__s1.pop())
        returnValue = self.__s1.pop()
        self.__s2.push(returnValue)
        while self.__s2.size() > 0:
            self.__s1.push(self.__s2.pop())
        return returnValue

    def empty(self) -> bool:
        if self.__s1.size() == 0:
            return True
        else:
            return False

# for brevity, some methods are omitted
class Stack(object):
    def __init__(self):
        self.__list = []

    # add an item to the stack
    def push(self, item):
        """
        add an item to the stack
        :param item: the item to be added
        :return: no return
        """
        self.__list.append(item)

    # remove and return the last added object
    def pop(self):
        if self.size() > 0:
            return self.__list.pop()
        else:
            return None

    # return the size of the stack
    def size(self):
        return len(self.__list)