[GP-165] Implemented tests for RB-tree

2-0-data-structures-and-algorithms/2-2-7-red-black-tree/README.md

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+# <img src="https://raw.githubusercontent.com/bobocode-projects/resources/master/image/logo_transparent_background.png" height=50/>Reb-Black Tree
+Learn a Red-Black tree data structure and build strong skills creating balanced tree implementation 💪
+
+### Objectives (at the end of the training you will be able to...)
+* understand the **main idea** behind the **Red-Black Tree** data structure ✅
+* get knowledge in one of the tree balancing strategies
+* implement rotations and recoloring algorithms
+* catch what is different from simple BinarySearchTree and why it's better
+* get stronger knowledge in algorithms field
+
+* The more about Red Black Tree can be found on [Wiki page](https://en.wikipedia.org/wiki/Red%E2%80%93black_tree)
+---
+#### 🆕 First time here? – [See Introduction](https://github.com/bobocode-projects/java-fundamentals-exercises/tree/main/0-0-intro#introduction)
+#### ➡️ Have any feedback? – [Please fill the form ](https://forms.gle/7U9XZHuTtT5xpjXR6)
+
+##
+<div align="center"><img src="https://raw.githubusercontent.com/bobocode-projects/resources/master/animation/GitHub%20Star_3.gif" height=50/></div>

2-0-data-structures-and-algorithms/2-2-7-red-black-tree/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>2-0-data-structures-and-algorithms</artifactId>
+        <groupId>com.bobocode</groupId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>2-2-7-red-black-tree</artifactId>
+
+
+</project>

2-0-data-structures-and-algorithms/2-2-7-red-black-tree/src/main/java/com/bobocode/cs/BinarySearchTree.java

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+package com.bobocode.cs;
+
+import java.util.function.Consumer;
+
+public interface BinarySearchTree<T extends Comparable<T>> {
+    /**
+     * insert an element
+     * @return true if element did not exist in the tree and was inserted successfully
+     */
+    boolean insert(T element);
+
+    /**
+     * @return true if tree contains element
+     */
+    boolean contains(T element);
+
+    /**
+     * @return number of elements in the tree
+     */
+    int size();
+
+    /**
+     * @return max. number of transition between root node and any other node; 0 - if tree is empty or contains 1 element
+     */
+    int depth();
+
+    /**
+     * traverse the tree in element's natural order
+     * @param consumer accepts ref. to node during traversing
+     */
+    void inOrderTraversal(Consumer<T> consumer);
+}

2-0-data-structures-and-algorithms/2-2-7-red-black-tree/src/main/java/com/bobocode/cs/RedBlackBinarySearchTree.java

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+package com.bobocode.cs;
+
+import java.util.function.Consumer;
+
+import com.bobocode.util.ExerciseNotCompletedException;
+
+/**
+ * {@link RedBlackBinarySearchTree} is an implementation of a {@link BinarySearchTree} that is based on nodes,
+ * coloring, rotations and recursion. A tree node is represented as a nested class {@link Node}. It holds an element (a value),
+ * references to the parent node, left/right child nodes and also a field type of {@link Color} which is Enum
+ * with two possible values RED and BLACK<br>
+ * The BinarySearch tree considered as a RedBlackTree when it meets next properties:<br>
+ *  1. Every {@link Node} is either RED or BLACK {@link Color}<br>
+ *  2. RED node can't have RED children nodes<br>
+ *  3. The root is always BLACK<br>
+ *  4. For each node, all simple paths from the node to descendant leaves(null nodes) contain the same number of BLACK nodes<br>
+ *  5. Every leaf(null node) is null and should be treated as a BLACK node<br>
+ *
+ * To fulfill these requirements the tree need additional steps after insertion of new node to re-balance itself using
+ * recoloring of nodes and rotations. Every newly inserted node should be RED before re-balancing.
+ *
+ * Red-Black Tree example:<br>
+ *.........................7(B).................................<br>
+ *..................../..........\..............................<br>
+ *.................../............\.............................<br>
+ *.................3(B)............10(R)........................<br>
+ *................/...\........../......\.......................<br>
+ *.............../.....\......../........\......................<br>
+ *.............2(R)....null...9(B)........12(B).................<br>
+ *............/....\........./....\....../....\.................<br>
+ *.........../......\......./......\..../......\................<br>
+ *.........null....null...null...null..null....33(R)............<br>
+ *............................................/....\............<br>
+ * ........................................../......\...........<br>
+ * ........................................null.....null........<br>
+ *
+ * <p><p>
+ * <strong>TODO: to get the most out of your learning, <a href="https://www.bobocode.com/learn">visit our website</a></strong>
+ * <p>
+ *
+ * @param <T> a type of elements that are stored in the tree
+ * @author Serhii Bondarenko
+ * @author Taras Boychuk
+ */
+
+public class RedBlackBinarySearchTree<T extends Comparable<T>> implements BinarySearchTree<T> {
+
+    public static <T extends Comparable<T>> RedBlackBinarySearchTree<T> of(T... elements) {
+        throw new ExerciseNotCompletedException();
+    }
+
+    @Override
+    public boolean insert(T element) {
+        throw new ExerciseNotCompletedException();
+    }
+
+    @Override
+    public boolean contains(T element) {
+       throw new ExerciseNotCompletedException();
+    }
+
+    @Override
+    public int size() {
+        throw new ExerciseNotCompletedException();
+    }
+
+    @Override
+    public int depth() {
+        throw new ExerciseNotCompletedException();
+    }
+
+    @Override
+    public void inOrderTraversal(Consumer<T> consumer) {
+        throw new ExerciseNotCompletedException();
+    }
+
+}