diff --git a/benchmarks/AbstractBench.php b/benchmarks/AbstractBench.php
index 6d5958f..f8212b4 100644
--- a/benchmarks/AbstractBench.php
+++ b/benchmarks/AbstractBench.php
@@ -11,6 +11,6 @@ abstract class AbstractBench
      */
     public function getData()
     {
-        return \range(1, 1000);
+        return \range(1, 100);
     }
 }
diff --git a/benchmarks/DrupolPhpTreeBench.php b/benchmarks/DrupolPhpTreeBench.php
index 6a66e94..519a3e8 100644
--- a/benchmarks/DrupolPhpTreeBench.php
+++ b/benchmarks/DrupolPhpTreeBench.php
@@ -30,12 +30,12 @@ public function initObject()
      * @Iterations(5)
      * @Warmup(10)
      */
-    public function benchHash()
+    public function benchTreeAdd()
     {
-        $this->tree = new ValueNode();
+        $this->tree = new ValueNode('root', 2);
 
         foreach ($this->getData() as $value) {
-            $this->tree->add(new ValueNode($value));
+            $this->tree->add(new ValueNode($value, 2));
         }
     }
 }
diff --git a/spec/drupol/phptree/Exporter/GraphSpec.php b/spec/drupol/phptree/Exporter/GraphSpec.php
index 850a9cd..50136aa 100644
--- a/spec/drupol/phptree/Exporter/GraphSpec.php
+++ b/spec/drupol/phptree/Exporter/GraphSpec.php
@@ -5,7 +5,6 @@
 namespace spec\drupol\phptree\Exporter;
 
 use drupol\phptree\Exporter\Graph;
-use drupol\phptree\Node\Node;
 use drupol\phptree\Node\ValueNode;
 use drupol\phptree\Traverser\BreadthFirst;
 use PhpSpec\ObjectBehavior;
@@ -20,9 +19,9 @@ public function it_is_initializable()
     public function it_can_generate_a_graph()
     {
         $root = new ValueNode('root');
-        $child1 = new Node();
-        $child2 = new Node();
-        $child3 = new Node();
+        $child1 = new ValueNode();
+        $child2 = new ValueNode();
+        $child3 = new ValueNode();
         $root
             ->add($child1, $child2, $child3);
 
diff --git a/spec/drupol/phptree/Exporter/SimpleArraySpec.php b/spec/drupol/phptree/Exporter/SimpleArraySpec.php
index 9e7e162..d1cf62f 100644
--- a/spec/drupol/phptree/Exporter/SimpleArraySpec.php
+++ b/spec/drupol/phptree/Exporter/SimpleArraySpec.php
@@ -25,7 +25,7 @@ public function it_can_export_to_an_array()
 
         $nodes = [];
         foreach (\range('A', 'J') as $value) {
-            $nodes[$value] = new ValueNode($value);
+            $nodes[$value] = new ValueNode($value, 2);
         }
 
         $tree->add(...\array_values($nodes));
diff --git a/spec/drupol/phptree/Exporter/TextSpec.php b/spec/drupol/phptree/Exporter/TextSpec.php
index ec22112..23e1478 100644
--- a/spec/drupol/phptree/Exporter/TextSpec.php
+++ b/spec/drupol/phptree/Exporter/TextSpec.php
@@ -25,7 +25,7 @@ public function it_can_export_to_text()
 
         $nodes = [];
         foreach (\range('A', 'J') as $value) {
-            $nodes[$value] = new ValueNode($value);
+            $nodes[$value] = new ValueNode($value, 2);
         }
 
         $tree->add(...\array_values($nodes));
diff --git a/spec/drupol/phptree/Node/NodeSpec.php b/spec/drupol/phptree/Node/NodeSpec.php
index 8186e14..b24b6d5 100644
--- a/spec/drupol/phptree/Node/NodeSpec.php
+++ b/spec/drupol/phptree/Node/NodeSpec.php
@@ -98,20 +98,6 @@ public function it_can_get_its_children()
             ->shouldYield(new \ArrayIterator([$node, $node]));
     }
 
-    public function it_can_get_the_last_children()
-    {
-        $this
-            ->lastChild()
-            ->shouldReturn(null);
-
-        $node = new Node();
-
-        $this
-            ->add($node)
-            ->lastChild()
-            ->shouldReturn($node);
-    }
-
     public function it_can_check_if_its_a_leaf()
     {
         $this
@@ -237,7 +223,7 @@ public function it_can_get_its_depth()
 
         $nodes = [];
         foreach (\range('A', 'Z') as $v) {
-            $nodes[] = new \drupol\phptree\Node\ValueNode($v);
+            $nodes[] = new \drupol\phptree\Node\ValueNode($v, 2);
         }
 
         $tree->add(...$nodes);
diff --git a/spec/drupol/phptree/Traverser/InOrderSpec.php b/spec/drupol/phptree/Traverser/InOrderSpec.php
index bdcd6be..01ca341 100644
--- a/spec/drupol/phptree/Traverser/InOrderSpec.php
+++ b/spec/drupol/phptree/Traverser/InOrderSpec.php
@@ -15,7 +15,7 @@ public function it_is_initializable()
         $this->shouldHaveType(InOrder::class);
     }
 
-    public function it_can_traverse_a_tree()
+    public function it_can_traverse_a_tree_of_degree2()
     {
         $tree = new ValueNode('root', 2);
 
@@ -39,4 +39,50 @@ public function it_can_traverse_a_tree()
             ->traverse($tree)
             ->shouldYield(new \ArrayIterator($nodes));
     }
+
+    public function it_can_traverse_a_tree_of_degree4()
+    {
+        $tree = new ValueNode('root', 4);
+
+        foreach (\range('A', 'Z') as $key => $value) {
+            $nodes[$value] = new ValueNode($value, 4);
+        }
+
+        $tree->add(...\array_values($nodes));
+
+        $nodes['root'] = $tree;
+        $nodes = [
+            $nodes['U'],
+            $nodes['V'],
+            $nodes['E'],
+            $nodes['W'],
+            $nodes['X'],
+            $nodes['Y'],
+            $nodes['F'],
+            $nodes['Z'],
+            $nodes['A'],
+            $nodes['G'],
+            $nodes['H'],
+            $nodes['I'],
+            $nodes['J'],
+            $nodes['B'],
+            $nodes['K'],
+            $nodes['L'],
+            $nodes['root'],
+            $nodes['M'],
+            $nodes['N'],
+            $nodes['C'],
+            $nodes['O'],
+            $nodes['P'],
+            $nodes['Q'],
+            $nodes['R'],
+            $nodes['D'],
+            $nodes['S'],
+            $nodes['T'],
+        ];
+
+        $this
+            ->traverse($tree)
+            ->shouldYield(new \ArrayIterator($nodes));
+    }
 }
diff --git a/src/Node/NaryNode.php b/src/Node/NaryNode.php
index 22bdc4c..99eccda 100644
--- a/src/Node/NaryNode.php
+++ b/src/Node/NaryNode.php
@@ -5,6 +5,7 @@
 namespace drupol\phptree\Node;
 
 use drupol\phptree\Traverser\BreadthFirst;
+use drupol\phptree\Traverser\TraverserInterface;
 
 /**
  * Class NaryNode.
@@ -18,6 +19,13 @@ class NaryNode extends Node
      */
     private $capacity;
 
+    /**
+     * The traverser.
+     *
+     * @var TraverserInterface
+     */
+    private $traverser;
+
     /**
      * NaryNode constructor.
      *
@@ -25,14 +33,18 @@ class NaryNode extends Node
      *   The maximum children a node can have.
      * @param \drupol\phptree\Node\NodeInterface|null $parent
      *   The parent.
+     * @param \drupol\phptree\Traverser\TraverserInterface|null $traverser
+     *   The traverser.
      */
-    public function __construct(int $capacity = 0, NodeInterface $parent = null)
+    public function __construct(int $capacity = 0, NodeInterface $parent = null, TraverserInterface $traverser = null)
     {
         parent::__construct($parent);
 
         $this->capacity = $capacity < 0 ?
             0:
             $capacity;
+
+        $this->traverser = $traverser ?? new BreadthFirst();
     }
 
     /**
@@ -49,26 +61,41 @@ public function capacity(): int
     public function add(NodeInterface ...$nodes): NodeInterface
     {
         foreach ($nodes as $node) {
-            $capacity = $this->capacity();
-
-            if (0 === $capacity || $this->degree() < $capacity) {
-                parent::add($node);
+            /** @var \drupol\phptree\Node\Node $parent */
+            $parent = $this->findFirstAvailableNode();
+            $parent->storage['children'][] = $node->setParent($parent);
+        }
 
-                continue;
-            }
+        return $this;
+    }
 
-            // @todo Find a way to get rid of this.
-            $traverser = new BreadthFirst();
+    /**
+     * {@inheritdoc}
+     */
+    public function getTraverser()
+    {
+        return $this->traverser;
+    }
 
-            foreach ($traverser->traverse($this) as $node_visited) {
-                if ($node_visited->degree() >= $capacity) {
-                    continue;
-                }
+    /**
+     * Find first node in the tree that could have a new children.
+     *
+     * @return \drupol\phptree\Node\NodeInterface
+     */
+    private function findFirstAvailableNode(): NodeInterface
+    {
+        $capacity = $this->capacity();
 
-                $node_visited->add($node);
+        foreach ($this->getTraverser()->traverse($this) as $node) {
+            if (\method_exists($node, 'capacity')) {
+                $capacity = $node->capacity();
+            }
 
-                break;
+            if ($node->degree() >= $capacity) {
+                continue;
             }
+
+            return $node;
         }
 
         return $this;
diff --git a/src/Node/NaryNodeInterface.php b/src/Node/NaryNodeInterface.php
index 2b91b95..0ff3b9f 100644
--- a/src/Node/NaryNodeInterface.php
+++ b/src/Node/NaryNodeInterface.php
@@ -4,6 +4,8 @@
 
 namespace drupol\phptree\Node;
 
+use drupol\phptree\Traverser\TraverserInterface;
+
 /**
  * Interface NaryNodeInterface
  */
@@ -16,4 +18,12 @@ interface NaryNodeInterface
      *   The node capacity.
      */
     public function capacity(): int;
+
+    /**
+     * Get the traverser in use.
+     *
+     * @return \drupol\phptree\Traverser\TraverserInterface
+     *   The traverser.
+     */
+    public function getTraverser(): TraverserInterface;
 }
diff --git a/src/Node/Node.php b/src/Node/Node.php
index fd590ad..21bd2a2 100644
--- a/src/Node/Node.php
+++ b/src/Node/Node.php
@@ -25,7 +25,7 @@ public function __construct(NodeInterface $parent = null)
     {
         $this->storage = [
             'parent' => $parent,
-            'children' => [],
+            'children' => new \ArrayObject(),
         ];
     }
 
@@ -35,7 +35,9 @@ public function __construct(NodeInterface $parent = null)
     public function add(NodeInterface ...$nodes): NodeInterface
     {
         foreach ($nodes as $node) {
-            $this->storage['children'][] = $node->setParent($this);
+            $this->storage['children']->append(
+                $node->setParent($this)
+            );
         }
 
         return $this;
@@ -46,11 +48,13 @@ public function add(NodeInterface ...$nodes): NodeInterface
      */
     public function remove(NodeInterface ...$nodes): NodeInterface
     {
-        $this->storage['children'] = \array_filter(
-            $this->storage['children'],
-            function ($child) use ($nodes) {
-                return !\in_array($child, $nodes, true);
-            }
+        $this->storage['children'] = new \ArrayObject(
+            \array_filter(
+                (array) $this->storage['children'],
+                function ($child) use ($nodes) {
+                    return !\in_array($child, $nodes, true);
+                }
+            )
         );
 
         return $this;
@@ -79,17 +83,7 @@ public function getParent(): ?NodeInterface
      */
     public function children(): \Traversable
     {
-        yield from new \ArrayIterator($this->storage['children']);
-    }
-
-    /**
-     * {@inheritdoc}
-     */
-    public function lastChild(): ?NodeInterface
-    {
-        return (false === $end = \end($this->storage['children'])) ?
-            null:
-            $end;
+        yield from $this->storage['children'];
     }
 
     /**
@@ -109,7 +103,7 @@ public function getAncestors(): \Traversable
      */
     public function isLeaf(): bool
     {
-        return [] === $this->storage['children'];
+        return 0 === $this->storage['children']->count();
     }
 
     /**
@@ -145,7 +139,7 @@ public function getSibblings(): \Traversable
      */
     public function degree(): int
     {
-        return \count($this->storage['children']);
+        return $this->storage['children']->count();
     }
 
     /**
@@ -169,7 +163,7 @@ public function count(): int
     public function withChildren(NodeInterface ...$nodes): NodeInterface
     {
         $clone = clone $this;
-        $clone->storage['children'] = [];
+        $clone->storage['children'] = new \ArrayObject();
 
         return [] === $nodes ?
             $clone:
diff --git a/src/Node/NodeInterface.php b/src/Node/NodeInterface.php
index 64422c0..8abb0eb 100644
--- a/src/Node/NodeInterface.php
+++ b/src/Node/NodeInterface.php
@@ -116,12 +116,4 @@ public function depth(): int;
      *   The new object.
      */
     public function withChildren(NodeInterface ...$nodes): NodeInterface;
-
-    /**
-     * Get the last child if any.
-     *
-     * @return \drupol\phptree\Node\NodeInterface|null
-     *   The last child if any, null otherwise.
-     */
-    public function lastChild(): ?NodeInterface;
 }
diff --git a/src/Traverser/InOrder.php b/src/Traverser/InOrder.php
index 6156cfb..7052fd9 100644
--- a/src/Traverser/InOrder.php
+++ b/src/Traverser/InOrder.php
@@ -31,8 +31,11 @@ public function traverse(NodeInterface $node): \Traversable
      */
     private function doTraverse(NodeInterface $node): \Traversable
     {
-        foreach ($node->children() as $child) {
-            if ($node->lastChild() === $child) {
+        $countChildren = $node->degree();
+        $middle = \floor($countChildren/2);
+
+        foreach ($node->children() as $key => $child) {
+            if ((int) $key === (int) $middle) {
                 yield $this->index => $node;
                 $this->index++;
             }