private static <V> void keySet(Set<String> set, TreeTrie<V> t) {
if (t != null) {
if (t._key != null) set.add(t._key);
for (int i = 0; i < INDEX; i++) {
if (t._nextIndex[i] != null) keySet(set, t._nextIndex[i]);
}
for (int i = t._nextOther.size(); i-- > 0; ) keySet(set, t._nextOther.get(i));
}
}
@Override
public Set<String> keySet() {
Set<String> keys = new HashSet<>();
keySet(keys, this);
return keys;
}