Queue Implementation using a Linked List

The problem. Build a queue on a singly linked list of ListNodes (each has a val and a next). Support enqueue(x), dequeue() (remove and return the front item, or -1 if empty), front() (the front item, or -1 if empty), isEmpty() and size(). A linked list never runs out of slots, so there is no capacity to manage.

Example

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

A linked list can only cheaply change things it holds a pointer to. So the questions are: which end is the front, which is the back, and which pointers do we keep?