---
changelog:
  category: Release
  version: 20260905.203314.000000000
date: '2026-09-05T20:33:14Z'
seo:
  description: >-
    Finished Sat, Sep 5, 2026, 4:33 PM ET: all 3 core problems closed, 3
    problems closed that day. Scheduled: Mon Sep 7, 2026 · Phase: Phase III —
    Hierarchical ·…
title: Topic 13 — Binary Search Tree
type: changelog
---
<!-- release-topics: #15 -->
Finished Sat, Sep 5, 2026, 4:33 PM ET: all 3 core problems closed, 3 problems closed that day.

**Scheduled:** Mon Sep 7, 2026 · **Phase:** Phase III — Hierarchical · **Topic:** #15 · **Builds on:** Topics 5 + 11

## Closed Sep 5 — the day the topic finished

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #111 | LC 98 · Validate BST | core | medium | Sep 5 | [py](https://github.com/prdlk/leetcode/blob/20260905.203314.000000000/work/1/Medium/Tree/98.validate-binary-search-tree.py) · [docs](https://lc.prd.lk/98) |
| #112 | LC 230 · Kth Smallest in BST | core | medium | Sep 5 | [py](https://github.com/prdlk/leetcode/blob/20260905.203314.000000000/work/1/Medium/Tree/230.kth-smallest-element-in-a-bst.py) · [docs](https://lc.prd.lk/230) |
| #113 | LC 235 · LCA of BST | core | medium | Sep 5 | [py](https://github.com/prdlk/leetcode/blob/20260905.203314.000000000/work/1/Medium/Tree/235.lowest-common-ancestor-of-a-binary-search-tree.py) · [docs](https://lc.prd.lk/235) |

## Open at release

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #114 | LC 105 · Build from Preorder + Inorder | optional | medium | — | — |
| #115 | LC 530 · Minimum Absolute Difference | optional | easy | — | — |
| #116 | LC 173 · BST Iterator | optional | medium | — | — |
| #117 | LC 108 · Sorted Array to BST | optional | easy | — | — |

---

`20260905.203314.000000000` is [timever](https://gist.github.com/twolfson/de1b004dd22536b8e668) for the UTC instant the core set completed; it tags 6c18301, the last main commit before it.