From 3c6da4d6c47f5f18a38b30cd4b0aa6271045dd25 Mon Sep 17 00:00:00 2001
From: LearnCodeZaid <zaid.git.code@example.com>
Date: Tue, 6 Oct 2026 16:56:10 +0530
Subject: [PATCH] fix(tree): set parent pointer in BinaryTreeNode.replaceChild

replaceChild swapped the child pointer but never updated
replacementNode.parent, leaving stale parent links after BST
removal of a node with one child (issue 2102) and breaking any
code walking parent pointers (issue 1102).

Adds regression tests for both issues.

Fixes trekhleb/javascript-algorithms#1102
Part of trekhleb/javascript-algorithms#2102
---
 src/data-structures/tree/BinaryTreeNode.js    |  2 ++
 .../__test__/BinarySearchTree.test.js         | 23 +++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/src/data-structures/tree/BinaryTreeNode.js b/src/data-structures/tree/BinaryTreeNode.js
index 44c9390e41..b04b065290 100644
--- a/src/data-structures/tree/BinaryTreeNode.js
+++ b/src/data-structures/tree/BinaryTreeNode.js
@@ -167,11 +167,13 @@ export default class BinaryTreeNode {
 
     if (this.left && this.nodeComparator.equal(this.left, nodeToReplace)) {
       this.left = replacementNode;
+      replacementNode.parent = this;
       return true;
     }
 
     if (this.right && this.nodeComparator.equal(this.right, nodeToReplace)) {
       this.right = replacementNode;
+      replacementNode.parent = this;
       return true;
     }
 
diff --git a/src/data-structures/tree/binary-search-tree/__test__/BinarySearchTree.test.js b/src/data-structures/tree/binary-search-tree/__test__/BinarySearchTree.test.js
index 105f94b0b5..0ae92f37f0 100644
--- a/src/data-structures/tree/binary-search-tree/__test__/BinarySearchTree.test.js
+++ b/src/data-structures/tree/binary-search-tree/__test__/BinarySearchTree.test.js
@@ -1,4 +1,5 @@
 import BinarySearchTree from '../BinarySearchTree';
+import BinarySearchTreeNode from '../BinarySearchTreeNode';
 
 describe('BinarySearchTree', () => {
   it('should create binary search tree', () => {
@@ -52,6 +53,28 @@ describe('BinarySearchTree', () => {
     expect(removed2).toBe(true);
   });
 
+  it('should keep parent pointers consistent when removing a node with one child (issue #2102)', () => {
+    const root = new BinarySearchTreeNode(8);
+    root.insert(3);
+    root.insert(6);
+    root.remove(3);
+    root.remove(6);
+    expect(root.right).toBeNull();
+    expect(root.left).toBeNull();
+  });
+
+  it('should re-parent the surviving child via replaceChild (issue #1102)', () => {
+    const bst = new BinarySearchTree();
+    bst.insert(10);
+    bst.insert(5);
+    bst.insert(3);
+    const node5 = bst.root.left;
+    bst.remove(5);
+    expect(bst.root.left.value).toBe(3);
+    expect(bst.root.left.parent).toBe(bst.root);
+    expect(node5.parent).toBeNull();
+  });
+
   it('should insert object values', () => {
     const nodeValueCompareFunction = (a, b) => {
       const normalizedA = a || { value: null };