Use Set instead of Map<*, Boolean> in DiffTreeTraversal

Author: ibbem

src/main/java/org/variantsync/diffdetective/diff/difftree/traverse/DiffTreeTraversal.java

@@ -3,7 +3,8 @@
 import org.variantsync.diffdetective.diff.difftree.DiffNode;
 import org.variantsync.diffdetective.diff.difftree.DiffTree;
 
-import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Set;
 import java.util.function.Consumer;
 
 /**
@@ -21,12 +22,12 @@
  * @author Paul Bittner
  */
 public class DiffTreeTraversal {
-    private final HashMap<Integer, Boolean> visited;
+    private final Set<Integer> visited;
     private final DiffTreeVisitor visitor;
 
     private DiffTreeTraversal(final DiffTreeVisitor visitor) {
         this.visitor = visitor;
-        this.visited = new HashMap<>();
+        this.visited = new HashSet<>();
     }
 
     /**
@@ -85,11 +86,6 @@ public void visitChildrenOf(final DiffNode subtree) {
      *         False if the node was already marked visited.
      */
     public boolean markAsVisited(final DiffNode node) {
-        final Integer id = node.getID();
-        if (!visited.containsKey(id)) {
-            visited.put(id, true);
-            return true;
-        }
-        return false;
+        return visited.add(node.getID());
     }
 }