Write a O(n) method valuesInLevelOrder() that
returns a list of the nodes of a binary tree in level-order. That
is, the method should return the root, then the nodes at depth 1,
followed by the nodes at depth 2, and so on. Your algorithm should
begin by putting the tree root on an initially empty queue. Then
dequeue a node, add it to the output, and enqueue its left and
right children (if they exist). Repeat until the queue is...