Stack Using Two Queues

Implement a last-in-first-out stack using only queue operations: add to the back, remove from the front, look at the front, check the size and check if empty.

Implement the MyStack class:

• push(x) pushes x onto the stack.
• pop() removes the top element and returns it.
• top() returns the top element without removing it.
• empty() returns true if the stack has no elements.

pop and top are only called on a non-empty stack. In JavaScript, treat an array as a queue by using only push and shift; in Python use collections.deque with append and popleft.