Fix binary tree node.

Author: trekhleb

src/data-structures/tree/BinaryTreeNode.js

@@ -31,18 +31,36 @@ export default class BinaryTreeNode {
   }
 
   setLeft(node) {
+    // Reset parent for left node since it is going to be detached.
+    if (this.left) {
+      this.left.parent = null;
+    }
+
+    // Attach new node to the left.
     this.left = node;
-    if (node) {
+
+    // Make current node to be a parent for new left one.
+    if (this.left) {
       this.left.parent = this;
     }
+
     return this;
   }
 
   setRight(node) {
+    // Reset parent for right node since it is going to be detached.
+    if (this.right) {
+      this.right.parent = null;
+    }
+
+    // Attach new node to the right.
     this.right = node;
+
+    // Make current node to be a parent for new right one.
     if (node) {
       this.right.parent = this;
     }
+
     return this;
   }