Implement Queue

Implement a queue of integers (first in, first out) in which every operation takes O(1) amortized time.

Implement the MyQueue class:

• MyQueue() creates an empty queue.
• enqueue(x) adds x at the back.
• dequeue() removes and returns the front element, or -1 if the queue is empty.
• front() returns the front element without removing it, or -1 if the queue is empty.
• isEmpty() returns true if the queue has no elements.
• size() returns the number of elements.

Removing from the front of an array with shift() or pop(0) is O(n), so it does not meet the requirement.