Add AVL Tree (rust-lang#186)

* feat(data_structures): add AVL tree

* feat(avl_tree): add is_empty and default methods

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Andrii Siriak <[email protected]>

Author: 3 people

DIRECTORY.md

@@ -7,6 +7,7 @@
     * [Rot13](https://github.com/TheAlgorithms/Rust/blob/master/src/ciphers/rot13.rs)
     * [Vigenere](https://github.com/TheAlgorithms/Rust/blob/master/src/ciphers/vigenere.rs)
   * Data Structures
+    * [Avl Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/avl_tree.rs)
     * [B Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/b_tree.rs)
     * [Binary Search Tree](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/binary_search_tree.rs)
     * [Graph](https://github.com/TheAlgorithms/Rust/blob/master/src/data_structures/graph.rs)

README.md

@@ -50,7 +50,7 @@ These are for demonstration purposes only.
 - [ ] Trie
 - [x] [Binary Search Tree](./src/data_structures/binary_search_tree.rs)
 - [x] [B-Tree](./src/data_structures/b_tree.rs)
-- [ ] AVL Tree
+- [x] [AVL Tree](./src/data_structures/avl_tree.rs)
 
 ## [Strings](./src/string)
 

src/data_structures/README.md

@@ -18,3 +18,19 @@ __Sources to read:__
 * [Rust API Docs](https://doc.rust-lang.org/std/collections/struct.BTreeMap.html)
 * [Keon Algorithms](https://github.com/keon/algorithms)
 * [MIT Open Course](https://www.youtube.com/watch?v=TOb1tuEZ2X4)
+
+### [AVL Tree](./avl_tree.rs)
+
+An AVL Tree is a self-balancing binary search tree. The heights of any two sibling
+nodes must differ by at most one; the tree may rebalance itself after insertion or
+deletion to uphold this property.
+
+__Properties__
+* Worst/Average time complexity for basic operations: O(log n)
+* Worst/Average space complexity: O(n)
+
+__Sources to read:__
+* [Wikipedia](https://en.wikipedia.org/wiki/AVL_tree)
+* Geeksforgeeks
+([Insertion](https://www.geeksforgeeks.org/avl-tree-set-1-insertion),
+[Deletion](https://www.geeksforgeeks.org/avl-tree-set-2-deletion))