Queue Implementation using Arrays

The problem. Build a queue on top of an array. Support enqueue(x) (add x at the back), dequeue() (remove and return the front item, or -1 if the queue is empty), front() (return the front item, or -1 if empty), isEmpty() and size(). The queue must keep working however many items pass through it.

Example

enqueue(10)   enqueue(20)   front()   -> 10
dequeue()     -> 10         size()    -> 1
dequeue()     -> 20         dequeue() -> -1  (empty)

Enqueue is easy on any array: append at the end. The whole problem is the front. Items leave from the start of the array, and the start is exactly where an array is slow to change.