#279 Stack From a Single Queue

A message bus only offers a first-in-first-out queue (add at the back, remove from the front, read the front, size), but a feature needs last-in-first-out behavior. Write a class QueueStack that uses one queue and only those operations:

• push(x) adds x.
• pop() removes and returns the most recently added item.
• top() returns the most recently added item.
• empty() returns whether the stack has no items.

pop and top are only called when the stack is not empty.

Each example lists the calls in order, then each call's arguments. The first call creates the object with new QueueStack(…); the output lists what each call returns (null for the constructor and for methods that return nothing).

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

Input: ["QueueStack","empty"]
[[],[]]
Output: [null,true]

Constraints

• 1 <= x <= 9
• At most 100 calls.