Use ArrayDeque instead of LinkedList for queue implementation

Use ArrayDeque instead of LinkedList for queue implementation where thread safety is not needed.

From https://docs.oracle.com/javase/7/docs/api/java/util/ArrayDeque.html

Resizable-array implementation of the Deque interface. Array deques have no capacity restrictions; they grow as necessary to support usage. They are not thread-safe; in the absence of external synchronization, they do not support concurrent access by multiple threads. Null elements are prohibited. This class is likely to be faster than Stack when used as a stack, and faster than LinkedList when used as a queue.