---
changelog:
  category: Release
  version: 20260923.194052.000000000
date: '2026-09-23T19:40:52Z'
seo:
  description: >-
    Finished Wed, Sep 23, 2026, 3:40 PM ET: all 2 core problems closed, 2
    problems closed that day. Scheduled: Wed Sep 23, 2026 · Phase: Phase IV —
    Relational ·…
title: Topic 19 — Union-Find
type: changelog
---
<!-- release-topics: #21 -->
Finished Wed, Sep 23, 2026, 3:40 PM ET: all 2 core problems closed, 2 problems closed that day.

**Scheduled:** Wed Sep 23, 2026 · **Phase:** Phase IV — Relational · **Topic:** #21 · **Builds on:** Topic 16's problem class

## Closed Sep 23 — the day the topic finished

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #146 | LC 684 · Redundant Connection | core | medium | Sep 23 | [py](https://github.com/prdlk/leetcode/blob/20260923.194052.000000000/work/1/Medium/Depth-First%20Search/684.redundant-connection.py) · [docs](https://lc.prd.lk/684) |
| #147 | LC 323 · Number of Connected Components | core | medium | Sep 23 | [py](https://github.com/prdlk/leetcode/blob/20260923.194052.000000000/work/1/Medium/Depth-First%20Search/323.number-of-connected-components-in-an-undirected-graph.py) · [docs](https://lc.prd.lk/323) |

## Open at release

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #148 | LC 721 · Accounts Merge | optional | medium | — | — |
| #217 | LC 547 · Number of Provinces | optional | medium | — | — |
| #220 | LC 1971 · Find if Path Exists in Graph | optional | easy | — | — |

---

`20260923.194052.000000000` is [timever](https://gist.github.com/twolfson/de1b004dd22536b8e668) for the UTC instant the core set completed; it tags ac4dc65, the last main commit before it.