---
changelog:
  category: Release
  version: 20260924.183638.000000000
date: '2026-09-24T18:36:38Z'
seo:
  description: >-
    Finished Thu, Sep 24, 2026, 2:36 PM ET: all 3 core problems closed, 3
    problems closed that day. Scheduled: Thu Sep 24, 2026 · Phase: Phase V —
    Decision Space…
title: Topic 20 — Backtracking
type: changelog
---
<!-- release-topics: #22 -->
Finished Thu, Sep 24, 2026, 2:36 PM ET: all 3 core problems closed, 3 problems closed that day.

**Scheduled:** Thu Sep 24, 2026 · **Phase:** Phase V — Decision Space · **Topic:** #22 · **Builds on:** Topic 16 — then unlocks LC 212 from Topic 15

## Closed Sep 24 — the day the topic finished

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #149 | LC 78 · Subsets | core | medium | Sep 24 | [py](https://github.com/prdlk/leetcode/blob/20260924.183638.000000000/work/1/Medium/Array/78.subsets.py) · [docs](https://lc.prd.lk/78) |
| #150 | LC 39 · Combination Sum | core | medium | Sep 24 | [py](https://github.com/prdlk/leetcode/blob/20260924.183638.000000000/work/1/Medium/Array/39.combination-sum.py) · [docs](https://lc.prd.lk/39) |
| #151 | LC 46 · Permutations | core | medium | Sep 24 | [py](https://github.com/prdlk/leetcode/blob/20260924.183638.000000000/work/1/Medium/Array/46.permutations.py) · [docs](https://lc.prd.lk/46) |

## Open at release

| Issue | Problem | Set | Difficulty | Closed (ET) | Solution |
| --- | --- | --- | --- | --- | --- |
| #152 | LC 79 · Word Search | optional | medium | — | — |
| #153 | LC 22 · Generate Parentheses | optional | medium | — | — |
| #154 | LC 17 · Letter Combinations | optional | medium | — | — |
| #155 | LC 77 · Combinations | optional | medium | — | — |
| #156 | LC 52 · N-Queens II | deferred | hard | — | — |
| #218 | LC 90 · Subsets II | optional | medium | — | — |
| #221 | LC 131 · Palindrome Partitioning | optional | medium | — | — |

---

`20260924.183638.000000000` is [timever](https://gist.github.com/twolfson/de1b004dd22536b8e668) for the UTC instant the core set completed; it tags f0f479d, the last main commit before it.