Implement Stack using Queues.py

'''
Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Notes:
You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
'''

class MyStack(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.a = []
        self.b = []

    def push(self, x):
        """
        Push element x onto stack.
        :type x: int
        :rtype: void
        """
        if not self.a:
            self.b.append(x)
        else:
            self.a.append(x)

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if self.a:
            while len(self.a) > 1:
                self.b.append(self.a.pop(0))
            return self.a.pop(0)
        elif self.b:
            while len(self.b) > 1:
                self.a.append(self.b.pop(0))
            return self.b.pop(0)
        else:
            return None
        

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if self.a:
            while len(self.a) > 1:
                self.b.append(self.a.pop(0))
            res = self.a.pop(0)
            self.b.append(res)
            return res
        elif self.b:
            while len(self.b) > 1:
                self.a.append(self.b.pop(0))
            res = self.b.pop(0)
            self.a.append(res)
            return res
        else:
            return None

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        return not self.a and not self.b


# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()