Tree parent and child nodes
Well, following conventions helps making your code more understandable. One convention is, for methods that return a boolean, using a predicate as a name, or as said in this answer, using a question. 

So to begin with, I'd name the method isAlreadyUsed.

A second improvement by following conventions would be using accessor methods:

node.getParent() instead of node.parent. But that depends on the scope of Node.

Item 14 in Joshua Bloch's Effective Java:


  In public classes, use accessor methods, not public fields