Fix binary tree node.

Author: trekhleb

src/data-structures/tree/BinaryTreeNode.js

@@ -26,6 +26,10 @@ export default class BinaryTreeNode {
     return Math.max(this.leftHeight, this.rightHeight);
   }
 
+  get balanceFactor() {
+    return this.leftHeight - this.rightHeight;
+  }
+
   setLeft(node) {
     this.left = node;
     this.left.parent = this;

src/data-structures/tree/__test__/BinaryTreeNode.test.js

@@ -115,13 +115,15 @@ describe('BinaryTreeNode', () => {
     const grandGrandLeft = new BinaryTreeNode(7);
 
     expect(root.height).toBe(0);
+    expect(root.balanceFactor).toBe(0);
 
     root
       .setLeft(left)
       .setRight(right);
 
     expect(root.height).toBe(1);
     expect(left.height).toBe(0);
+    expect(root.balanceFactor).toBe(0);
 
     left
       .setLeft(grandLeft)
@@ -131,6 +133,7 @@ describe('BinaryTreeNode', () => {
     expect(left.height).toBe(1);
     expect(grandLeft.height).toBe(0);
     expect(grandRight.height).toBe(0);
+    expect(root.balanceFactor).toBe(1);
 
     grandLeft.setLeft(grandGrandLeft);
 
@@ -139,6 +142,7 @@ describe('BinaryTreeNode', () => {
     expect(grandLeft.height).toBe(1);
     expect(grandRight.height).toBe(0);
     expect(grandGrandLeft.height).toBe(0);
+    expect(root.balanceFactor).toBe(2);
   });
 
   it('should calculate node height for right nodes as well', () => {
@@ -149,5 +153,6 @@ describe('BinaryTreeNode', () => {
 
     expect(root.height).toBe(1);
     expect(right.height).toBe(0);
+    expect(root.balanceFactor).toBe(-1);
   });
 });