# Recursion and Backtracking — Problem Tracker

[অধ্যায়ে ফিরে যাও](../README.md) · [Pattern guide](../patterns.md) · [চিত্রসহ ব্যাখ্যা](../visual-explanation.md)

**এই তালিকার প্রতিটি problem-এর বাংলা নোট ইতিমধ্যেই লেখা আছে।** `Official task` বা platform link দিয়ে মূল প্রশ্ন ও judge খুলবে; `বাংলা নোট` বা `Note file` দিয়ে এখানকার ব্যাখ্যা খুলবে। `planned` হলো তোমার ব্যক্তিগত practice status, নোট অনুপস্থিত থাকার চিহ্ন নয়। Difficulty/স্তর হলো এই পাঠক্রমের শেখার ক্রম; platform-এর difficulty আলাদা হতে পারে।

Problem-গুলো শুধু official link দিয়ে reference করা; নিজের note-এ প্রতিটা task নিজের ভাষায় লেখো।

| # | Problem | Difficulty | Source | Pattern | Inherits from | Note file | Status |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 1 | Fibonacci Number — <https://leetcode.com/problems/fibonacci-number/> | Easy | LeetCode | Decrease-and-conquer → memoization | Loops; recursion tree-র ছবি | [001-fibonacci-number.md](001-fibonacci-number.md) | planned |
| 2 | Climbing Stairs — <https://leetcode.com/problems/climbing-stairs/> | Easy | LeetCode | Decrease-and-conquer → memoization | ছদ্মবেশে Fibonacci | [002-climbing-stairs.md](002-climbing-stairs.md) | planned |
| 3 | Reverse String — <https://leetcode.com/problems/reverse-string/> | Easy | LeetCode | Decrease-and-conquer (two ends) | Swap + ভেতরের দিকে recurse | [003-reverse-string.md](003-reverse-string.md) | planned |
| 4 | Power of Two — <https://leetcode.com/problems/power-of-two/> | Easy | LeetCode | Decrease-and-conquer (halving) | Divisibility (math fundamentals) | [004-power-of-two.md](004-power-of-two.md) | planned |
| 5 | Pow(x, n) — <https://leetcode.com/problems/powx-n/> | Medium | LeetCode | Divide-and-conquer | concept.md-র fast power | [005-pow-x-n.md](005-pow-x-n.md) | planned |
| 6 | Merge Two Sorted Lists — <https://leetcode.com/problems/merge-two-sorted-lists/> | Easy | LeetCode | Decrease-and-conquer on lists | Linked lists section + leap of faith | [006-merge-two-sorted-lists.md](006-merge-two-sorted-lists.md) | planned |
| 7 | Subsets — <https://leetcode.com/problems/subsets/> | Medium | LeetCode | Include/exclude branch | Decrease-and-conquer, এবার branching সহ | [007-subsets.md](007-subsets.md) | planned |
| 8 | Letter Combinations of a Phone Number — <https://leetcode.com/problems/letter-combinations-of-a-phone-number/> | Medium | LeetCode | Cartesian choose-explore | Subsets branching, k-way | [008-letter-combinations-phone.md](008-letter-combinations-phone.md) | planned |
| 9 | Permutations — <https://leetcode.com/problems/permutations/> | Medium | LeetCode | Choose each remaining | Subsets + seen-set (../../05-hashing/) | [009-permutations.md](009-permutations.md) | planned |
| 10 | Subsets II — <https://leetcode.com/problems/subsets-ii/> | Medium | LeetCode | Include/exclude + duplicate skip | Subsets + sort-and-skip | [010-subsets-ii.md](010-subsets-ii.md) | planned |
| 11 | Combinations — <https://leetcode.com/problems/combinations/> | Medium | LeetCode | Start-index combinations | Fixed size k সহ subsets | [011-combinations.md](011-combinations.md) | planned |
| 12 | Combination Sum — <https://leetcode.com/problems/combination-sum/> | Medium | LeetCode | Combinations with pruning | Combinations + remaining-target complement | [012-combination-sum.md](012-combination-sum.md) | planned |
| 13 | Combination Sum II — <https://leetcode.com/problems/combination-sum-ii/> | Medium | LeetCode | Pruning + duplicate handling | Combination Sum + Subsets II-র skip trick | [013-combination-sum-ii.md](013-combination-sum-ii.md) | planned |
| 14 | Permutations II — <https://leetcode.com/problems/permutations-ii/> | Medium | LeetCode | Choose remaining + duplicate skip | Permutations + sorted skip | [014-permutations-ii.md](014-permutations-ii.md) | planned |
| 15 | Generate Parentheses — <https://leetcode.com/problems/generate-parentheses/> | Medium | LeetCode | Constraint backtracking (open/close counts) | Subsets branching + validity pruning | [015-generate-parentheses.md](015-generate-parentheses.md) | planned |
| 16 | Palindrome Partitioning — <https://leetcode.com/problems/palindrome-partitioning/> | Medium | LeetCode | Cut-point backtracking | Combinations (cut position-গুলোই choice) | [016-palindrome-partitioning.md](016-palindrome-partitioning.md) | planned |
| 17 | Word Search — <https://leetcode.com/problems/word-search/> | Medium | LeetCode | Grid backtracking, visited marks | Constraint backtracking + cell-এ seen-set | [017-word-search.md](017-word-search.md) | planned |
| 18 | Creating Strings — <https://cses.fi/problemset/task/1622/> (CSES, task name: Creating Strings) | Medium | CSES | Permutations with duplicates | CP-গতিতে Permutations II | [018-cses-creating-strings.md](018-cses-creating-strings.md) | planned |
| 19 | N-Queens — <https://leetcode.com/problems/n-queens/> | Hard | LeetCode | Constraint backtracking | Permutations + তিনটা blocked-line set | [019-n-queens.md](019-n-queens.md) | planned |
| 20 | N-Queens II — <https://leetcode.com/problems/n-queens-ii/> | Hard | LeetCode | Constraint backtracking (count only) | N-Queens, board-গুলো বাদ দাও | [020-n-queens-ii.md](020-n-queens-ii.md) | planned |
| 21 | Chessboard and Queens — <https://cses.fi/problemset/task/1624/> (CSES, task name: Chessboard and Queens) | Hard | CSES | N-Queens with blocked cells | N-Queens + বাড়তি constraint | [021-cses-chessboard-and-queens.md](021-cses-chessboard-and-queens.md) | planned |
| 22 | Sudoku Solver — <https://leetcode.com/problems/sudoku-solver/> | Hard | LeetCode | Constraint backtracking | N-Queens-এর শৃঙ্খলা, প্রতি cell-এ 3টা unit-set | [022-sudoku-solver.md](022-sudoku-solver.md) | planned |
| 23 | Word Search II — <https://leetcode.com/problems/word-search-ii/> | Hard | LeetCode | Backtracking + trie preview | Word Search; trie আসবে পরের এক section-এ | [023-word-search-ii.md](023-word-search-ii.md) | planned |

## এই tracker কীভাবে ব্যবহার করবে

- Status-গুলো: `planned` → `attempted` → `solved` → `revisit` (এক সপ্তাহ পরে cold অবস্থায় আবার solve করেছ)।
- প্রতিটা backtracking problem-এর note-এ থাকতেই হবে: (a) প্রতি level-এ decision-টা কী, (b) কী choose/un-choose হচ্ছে, (c) pruning condition-টা কী। এই তিনটি নিজের ভাষায় বলতে পারলে বোঝা যাবে state বদলানোর যুক্তি পরিষ্কার হয়েছে।
- Problem 7, 9, 12, আর 19 হলো চারটা canonical interview shape — ওই note-গুলোতে সবচেয়ে বেশি invest করো।
- 1-2 শেষ করার পরপরই `@lru_cache` দিয়ে আবার করো আর call-count-এর পার্থক্যটা লিখে রাখো; ওই observation-টাই [../../12-dynamic-programming/](../../12-dynamic-programming/README.md)-এর entry ticket।
- পুরো note format: [../../templates/ds-problem-note-template.md](../../templates/ds-problem-note-template.md)-র 24-section template।