Add a NodeInterface::height() method and its tests.

spec/drupol/phptree/Node/NodeSpec.php

@@ -234,4 +234,37 @@ public function it_can_get_its_depth()
             ->depth()
             ->shouldReturn(4);
     }
+
+    public function it_can_get_its_height()
+    {
+        $this
+            ->height()
+            ->shouldReturn(0);
+
+        $tree = $this;
+        foreach (\range('A', 'B') as $key => $v) {
+            $node = new \drupol\phptree\Node\ValueNode($v, 1);
+            $tree->add($node);
+            $tree = $node;
+        }
+
+        $this
+            ->height()
+            ->shouldReturn(2);
+
+        foreach (\range('C', 'F') as $key => $v) {
+            $node = new \drupol\phptree\Node\ValueNode($v, 1);
+            $tree->add($node);
+            $tree = $node;
+        }
+
+        $this
+            ->height()
+            ->shouldReturn(6);
+
+        $this
+            ->withChildren()
+            ->height()
+            ->shouldReturn(0);
+    }
 }

src/Node/Node.php

@@ -177,4 +177,18 @@ public function depth(): int
     {
         return \iterator_count($this->getAncestors());
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function height(): int
+    {
+        $height = $this->depth();
+
+        foreach ($this->children() as $child) {
+            $height = \max($height, $child->height());
+        }
+
+        return $height;
+    }
 }

src/Node/NodeInterface.php

@@ -106,6 +106,14 @@ public function degree(): int;
      */
     public function depth(): int;
 
+    /**
+     * Get the tree height.
+     *
+     * @return int
+     *   The tree height.
+     */
+    public function height(): int;
+
     /**
      * Get a clone of the object with or without children.
      *