lookup method
override
If an object equal to object is in the set, return it.
Checks if there is an object in the set that is equal to object.
If so, that object is returned, otherwise returns null.
Implementation
E lookup(Object object) {
if (!_validKey(object)) return null;
int comp = _splay(object);
if (comp != 0) return null;
return _root.key;
}