Add MirrorTree task solution

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

@@ -0,0 +1,31 @@
+package by.andd3dfx.tree;
+
+import lombok.AllArgsConstructor;
+
+/**
+ * Mirror binary tree: for each node left & right sub nodes should be swapped
+ */
+public class MirrorTree {
+
+    public static Node mirror(Node node) {
+        if (node == null) {
+            return null;
+        }
+
+        var oldLeft = mirror(node.left);
+        node.left = mirror(node.right);
+        node.right = oldLeft;
+        return node;
+    }
+
+    @AllArgsConstructor
+    public static class Node {
+        int value;
+        Node left;
+        Node right;
+
+        public Node(int value) {
+            this.value = value;
+        }
+    }
+}

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

@@ -0,0 +1,51 @@
+package by.andd3dfx.tree;
+
+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 {
+
+    /**
+     * <pre>
+     *        4                  4
+     *   3        5    =>    5      3
+     * 1  2     6   7      7  6    2  1
+     *     8   9 0           0 9  8
+     *   -2                        -2
+     * </pre>
+     */
+    @Test
+    public void testMirror() {
+        var _m2 = new Node(-2);
+        var _8 = new Node(8, _m2, null);
+        var _9 = new Node(9);
+        var _0 = new Node(0);
+        var _1 = new Node(1);
+        var _2 = new Node(2, null, _8);
+        var _6 = new Node(6, _9, _0);
+        var _7 = new Node(7);
+        var _3 = new Node(3, _1, _2);
+        var _5 = new Node(5, _6, _7);
+        var _4 = new Node(4, _3, _5);
+
+        var result = mirror(_4);
+
+        assertThat(result).isEqualTo(_4);
+        check(_4, _5, _3);
+        check(_5, _7, _6);
+        check(_3, _2, _1);
+        check(_7, null, null);
+        check(_6, _0, _9);
+        check(_2, _8, null);
+        check(_1, null, null);
+        check(_8, null, _m2);
+    }
+
+    private void check(Node node, Node expectedLeft, Node expectedRight) {
+        assertThat(node.left).isEqualTo(expectedLeft);
+        assertThat(node.right).isEqualTo(expectedRight);
+    }
+}