make unionUnsorted 5-10% faster for lists with 100k elements

primitiveCollections/src/main/java/org/lucares/collections/IntList.java

@@ -1062,14 +1062,14 @@ public final class IntList implements Serializable, Cloneable {
 	}
 
 	private static IntList unionUnsorted(final IntList a, final IntList b) {
-		// TODO use a more efficient algorithm. Especially the removeIf is too
-		// expensive, because of all the method calls
-		final IntList result;
-		result = new IntList(a.size() + b.size());
-		result.addAll(a);
-		result.addAll(b);
-		result.sort();
-		result.removeIf((value, index) -> index > 0 && result.get(index) == result.get(index - 1));
-		return result;
+		// TODO use a more efficient algorithm. The sort operations make this 10 times
+		// slower than unionSorted
+
+		final IntList aSorted = new IntList(a);
+		aSorted.parallelSort();
+		final IntList bSorted = new IntList(b);
+		bSorted.parallelSort();
+
+		return unionSorted(aSorted, bSorted);
 	}
 }