Fix binary tree node.

src/data-structures/tree/BinaryTreeNode.js

@@ -32,13 +32,17 @@ export default class BinaryTreeNode {
 
   setLeft(node) {
     this.left = node;
-    this.left.parent = this;
+    if (node) {
+      this.left.parent = this;
+    }
     return this;
   }
 
   setRight(node) {
     this.right = node;
-    this.right.parent = this;
+    if (node) {
+      this.right.parent = this;
+    }
     return this;
   }
 

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

@@ -155,4 +155,22 @@ describe('BinaryTreeNode', () => {
     expect(right.height).toBe(0);
     expect(root.balanceFactor).toBe(-1);
   });
+
+  it('should set null for left and right node', () => {
+    const root = new BinaryTreeNode(2);
+    const left = new BinaryTreeNode(1);
+    const right = new BinaryTreeNode(3);
+
+    root.setLeft(left);
+    root.setRight(right);
+
+    expect(root.left.value).toBe(1);
+    expect(root.right.value).toBe(3);
+
+    root.setLeft(null);
+    root.setRight(null);
+
+    expect(root.left).toBeNull();
+    expect(root.right).toBeNull();
+  });
 });