BSA — DSA Patterns & Solutions

A curated Data Structures & Algorithms (DSA) repository focused on problem-solving patterns (not random problem dumps).
Each pattern includes intuition, templates, pitfalls, and solutions with time/space complexity so you can learn once and reuse everywhere.

---

What you’ll find here

• Pattern-first learning: Binary Search, Two Pointers, Sliding Window, DFS/BFS, DP, Greedy, Backtracking, etc.
• Clean solutions: readable, consistent style, and interview-friendly.
• Explanations that teach: why the approach works, not just what to type.
• Complexity notes: time and space called out for each solution.
• Reusable templates: minimal “starter” code to apply patterns quickly.

---

How to navigate this repo (fast)

1. Start in the Patterns section below.
2. Open the folder path for the pattern you want.
3. Read that folder’s README.md for:
   • when to use the pattern
   • core idea / invariants
   • template
   • common mistakes
   • curated problems + solutions in that folder

Tip: If you’re preparing for interviews, pick 1–2 patterns per week, then solve 10–20 problems per pattern.

---

Patterns (directory index)

Below are the main pattern “packages” (folder paths). Each folder should contain a README.md explanation page plus solutions.

Searching & Ordering

• patterns/binary-search/
• patterns/two-pointers/
• patterns/sliding-window/
• patterns/merge-intervals/

Graphs & Trees

• patterns/bfs/
• patterns/dfs/
• patterns/topological-sort/
• patterns/union-find/
• patterns/shortest-path/

Dynamic Programming (DP)

• patterns/dp-1d/
• patterns/dp-2d/
• patterns/dp-knapsack/
• patterns/dp-subsequence/

Backtracking & Recursion

• patterns/backtracking/
• patterns/recursion/

Greedy & Heaps

• patterns/greedy/
• patterns/heap-priority-queue/

Data Structures

• patterns/stack/
• patterns/queue/
• patterns/hashmap/
• patterns/trie/

Math & Bit

• patterns/math/
• patterns/bit-manipulation/

You can rename/reorder these based on your preferred curriculum.

---

Repo structure (suggested)

This is a clean structure that scales as the repo grows:

• patterns/
  • binary-search/
    • README.md (pattern explanation + templates + pitfalls + problem list)
    • solutions/ (or keep solutions in the folder root)
  • sliding-window/
  • dp-1d/
• templates/ (optional: cross-pattern templates)
• notes/ (optional: general notes, cheat sheets)
• README.md (this file)

---

Conventions used

To keep solutions consistent and easy to review, this repo follows these conventions:

• File naming

  • Use descriptive names: lower_bound, first_true, rotated_sorted_array, etc.
  • If using numbered platforms, add a suffix: two_sum_leetcode, aggressive_cows_gfg, etc.

• Solution format (recommended)

  • Problem summary (1–3 lines)
  • Key idea / invariant
  • Complexity
  • Implementation

• Complexity notation

  • Time: O(...)
  • Space: O(...) (state if auxiliary space vs total space matters)

---

Quality bar (what “done” means)

A solution is considered “complete” when it has:

• A correct approach (handles edge cases)
• Clear variable names
• Complexity stated
• At least one of:
  • brief proof/intuition, or
  • invariants explained, or
  • “why this pattern applies” note

---

Pattern templates (what I optimize for)

This repo emphasizes templates you can memorize and adapt, such as:

• Binary Search on answer (first true / last true)
• Sliding Window (fixed vs variable window)
• BFS level-order
• DFS recursion with visited/parent
• DP state + transition + base cases
• Backtracking (choose → explore → unchoose)

Templates may live inside each pattern folder and/or in templates/.

---

Suggested learning path (if you’re starting)

A practical progression that builds confidence quickly:

1. Arrays / Strings basics + Hashing
2. Two Pointers + Sliding Window
3. Binary Search (index) → Binary Search (answer space)
4. Stack / Monotonic Stack
5. Trees (DFS/BFS)
6. Graphs (BFS/DFS → Topo Sort → Union-Find)
7. Heaps + Greedy
8. Dynamic Programming (1D → 2D → classic patterns)
9. Backtracking (subsets/permutations/combinations)

---

Progress tracker (optional)

If you want a simple accountability system, track it here:

• Binary Search — core templates completed
• Sliding Window — fixed + variable window completed
• Two Pointers — classic patterns completed
• BFS/DFS — templates + 10 problems each
• DP — 1D + 2D + knapsack basics

(If you don’t want checkboxes, remove this section.)

---

Contributing

If you’re using this repo personally, treat this as your rulebook:

• Add problems only if they reinforce a pattern.
• Prefer fewer, higher-quality solutions over many rushed ones.
• If you revisit a problem and find a cleaner solution, replace it and add a short note on what improved.

If you plan to accept external contributions later, you can add:

• style rules
• required format
• PR checklist
• code review expectations

---

License

Add a license if you plan to share widely (MIT is a common choice).
If you’re unsure, you can leave this out until you decide.

---

Contact / Notes

This repo is built as a living notebook. If something is unclear:

• check the pattern README.md
• compare multiple solutions in the same pattern
• add a short note to improve future-you’s understanding