Fix issue trekhleb#37 with AVL tree.

Author: trekhleb

src/data-structures/tree/avl-tree/AvlTree.js

@@ -80,6 +80,12 @@ export default class AvlTree extends BinarySearchTree {
     const leftRightNode = leftNode.right;
     leftNode.setRight(null);
 
+    // Preserve leftRightNode's left subtree.
+    if (leftRightNode.left) {
+      leftNode.setRight(leftRightNode.left);
+      leftRightNode.setLeft(null);
+    }
+
     // Attach leftRightNode to the rootNode.
     rootNode.setLeft(leftRightNode);
 
@@ -102,6 +108,11 @@ export default class AvlTree extends BinarySearchTree {
     const rightLeftNode = rightNode.left;
     rightNode.setLeft(null);
 
+    if (rightLeftNode.right) {
+      rightNode.setLeft(rightLeftNode.right);
+      rightLeftNode.setRight(null);
+    }
+
     // Attach rightLeftNode to the rootNode.
     rootNode.setRight(rightLeftNode);
 

src/data-structures/tree/avl-tree/README.md

@@ -45,3 +45,4 @@ AVL tree with balance factors (green)
 * [Tutorials Point](https://www.tutorialspoint.com/data_structures_algorithms/avl_tree_algorithm.htm)
 * [BTech](http://btechsmartclass.com/DS/U5_T2.html)
 * [AVL Tree Insertion on YouTube](https://www.youtube.com/watch?v=rbg7Qf8GkQ4&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8&index=12&)
+* [AVL Tree Operations Visualisations](https://www.cs.usfca.edu/~galles/visualization/AVLtree.html)

src/data-structures/tree/avl-tree/__test__/AvlTRee.test.js

@@ -185,4 +185,48 @@ describe('AvlTree', () => {
     expect(tree.root.height).toBe(2);
     expect(tree.toString()).toBe('6,8,9,18,21,22,43');
   });
+
+  it('should do left right rotation and keeping left right node safe', () => {
+    const tree = new AvlTree();
+
+    tree.insert(30);
+    tree.insert(15);
+    tree.insert(40);
+    tree.insert(10);
+    tree.insert(18);
+    tree.insert(35);
+    tree.insert(45);
+    tree.insert(5);
+    tree.insert(12);
+
+    expect(tree.toString()).toBe('5,10,12,15,18,30,35,40,45');
+    expect(tree.root.height).toBe(3);
+
+    tree.insert(11);
+
+    expect(tree.toString()).toBe('5,10,11,12,15,18,30,35,40,45');
+    expect(tree.root.height).toBe(3);
+  });
+
+  it('should do left right rotation and keeping left right node safe', () => {
+    const tree = new AvlTree();
+
+    tree.insert(30);
+    tree.insert(15);
+    tree.insert(40);
+    tree.insert(10);
+    tree.insert(18);
+    tree.insert(35);
+    tree.insert(45);
+    tree.insert(42);
+    tree.insert(47);
+
+    expect(tree.toString()).toBe('10,15,18,30,35,40,42,45,47');
+    expect(tree.root.height).toBe(3);
+
+    tree.insert(43);
+
+    expect(tree.toString()).toBe('10,15,18,30,35,40,42,43,45,47');
+    expect(tree.root.height).toBe(3);
+  });
 });