fix(tree): set parent pointer in BinaryTreeNode.replaceChild

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;
     }
 

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 };