Fix tail update on insertion in CircularLinkedList

DataStructures.Tests/LinkedList/CircularLinkedListTests.cs

@@ -175,4 +175,20 @@ private static string GetDisplayOutput<T>(CircularLinkedList<T> list)
 
         return result.ToString().Trim();
     }
+
+    [Test]
+    public static void InsertAfter_InsertingAfterTail_UpdatesTailAndPreservesHead()
+    {
+        var list = new CircularLinkedList<int>();
+        list.InsertAtEnd(10);
+
+        list.InsertAfter(10, 20);
+
+        var head = list.GetHead();
+        Assert.That(head, Is.Not.Null);
+        Assert.That(head!.Data, Is.EqualTo(10));
+        Assert.That(head.Next, Is.Not.Null);
+        Assert.That(head.Next!.Data, Is.EqualTo(20));
+        Assert.That(head.Next.Next, Is.SameAs(head));
+    }
 }

DataStructures/LinkedList/CircularLinkedList/CircularLinkedList.cs

@@ -97,6 +97,11 @@ public void InsertAfter(T value, T data)
                     newNode.Next = current.Next;
                     current.Next = newNode;
 
+                    if (tail! == current)
+                    {
+                        tail = newNode;
+                    }
+
                     return;
                 }