MirrorTree task: add YT link, adjust solution+test

Author: andrei-punko

README.md

@@ -253,6 +253,7 @@ they contain enough code which describes implementation in a natural way.
 | Определение высоты бинарного дерева                                                              | [Youtube](https://youtu.be/xTftgqH0WVI) | [Code](src/main/java/by/andd3dfx/tree/TreeHeight.java)                                                                                                                              |
 | Dummy, Fake, Stub, Spy, Mock                                                                     | [Youtube](https://youtu.be/KkXqW8vMD-4) | -                                                                                                                                                                                   |
 | Нагрузочное тестирование Spring Boot сервиса с помощью Gatling                                   | [Youtube](https://youtu.be/uO9luM8wowo) | [Repo](https://github.com/andrei-punko/articles-backend-app)                                                                                                                        |
+| Отражение бинарного дерева                                                                       | [Youtube](https://youtu.be/H2BBCUcVXDM) | [Code](src/main/java/by/andd3dfx/tree/MirrorTree.java)                                                                                                                              |
 
 ## Materials & notes
 

src/main/java/by/andd3dfx/tree/MirrorTree.java

@@ -4,6 +4,8 @@
 
 /**
  * Mirror binary tree: for each node left & right sub nodes should be swapped
+ *
+ * @see <a href="https://youtu.be/H2BBCUcVXDM">Video solution</a>
  */
 public class MirrorTree {
 
@@ -12,9 +14,9 @@ public static Node mirror(Node node) {
             return null;
         }
 
-        var oldLeft = mirror(node.left);
+        var oldLeft = node.left;
         node.left = mirror(node.right);
-        node.right = oldLeft;
+        node.right = mirror(oldLeft);
         return node;
     }
 

src/test/java/by/andd3dfx/tree/MirrorTreeTest.java

@@ -3,7 +3,6 @@
 import by.andd3dfx.tree.MirrorTree.Node;
 import org.junit.Test;
 
-import static by.andd3dfx.tree.MirrorTree.mirror;
 import static org.assertj.core.api.Assertions.assertThat;
 
 public class MirrorTreeTest {
@@ -31,7 +30,7 @@ public void testMirror() {
         var _5 = new Node(5, _6, _7);
         var _4 = new Node(4, _3, _5);
 
-        var result = mirror(_4);
+        var result = MirrorTree.mirror(_4);
 
         assertThat(result).isEqualTo(_4);
         check(_4, _5, _3);
@@ -41,6 +40,8 @@ public void testMirror() {
         check(_6, _0, _9);
         check(_2, _8, null);
         check(_1, null, null);
+        check(_0, null, null);
+        check(_9, null, null);
         check(_8, null, _m2);
     }