Data Structures and Algorithms

A structured repository for learning data structures, algorithms, interview patterns, and high-signal LeetCode practice from first principles.

Purpose

This repository is designed to help a beginner become interview-ready without memorizing solution dumps. The goal is to build durable intuition, pattern recognition, edge-case discipline, and precise complexity analysis.

The curation favors the pattern coverage of NeetCode 150, Blind 75, Grind 75, and classic high-frequency interview problems. It also adds targeted extras where a topic needs more practice than the major public lists provide.

Repository Quality Standard

Every topic should answer five questions for a learner and for an interviewer:

• What is the core intuition?
• What invariant makes the pattern correct?
• Which examples and edge cases expose the pattern?
• Which mistakes cause wrong answers in interviews?
• Which problems build the skill without duplicating practice entries elsewhere?

Each problem appears once in the repository. When a problem is relevant to more than one topic, it is assigned to the topic where the dominant interview pattern is most useful.

Audit Lenses

• Google interviewer: looks for invariants, proof sketches, and clean complexity analysis.
• Meta interviewer: looks for fast pattern recognition, concise code, and reliable edge-case execution.
• Amazon interviewer: looks for clarification, tradeoffs, operational thinking, and practical communication.
• OpenAI, Anthropic, and Mistral engineer: look for correctness under changed constraints, careful reasoning, and simple maintainable logic.
• Beginner learner: needs plain-language intuition, diagrams, deliberate practice order, and mistakes called out before they happen.

How To Use This Repository

1. Read the topic README.md before solving.
2. Read PATTERNS.md and write each invariant in your own words.
3. Use CHEATSHEET.md for quick recall.
4. Solve Easy problems to learn mechanics.
5. Solve Medium problems to build interview fluency.
6. Use Hard problems selectively for advanced recognition and follow-up depth.
7. Keep a separate mistake log with pattern, missed invariant, edge case, and retry date.

Mermaid Roadmap

flowchart TD
    arrays_hashing["Arrays & Hashing"]
    two_pointers["Two Pointers"]
    stack["Stack"]
    binary_search["Binary Search"]
    sliding_window["Sliding Window"]
    linked_list["Linked List"]
    trees["Trees"]
    tries["Tries"]
    heap_priority_queue["Heap / Priority Queue"]
    backtracking["Backtracking"]
    graphs["Graphs"]
    advanced_graphs["Advanced Graphs"]
    one_d_dp["1-D Dynamic Programming"]
    two_d_dp["2-D Dynamic Programming"]
    greedy["Greedy"]
    intervals["Intervals"]
    bit_manipulation["Bit Manipulation"]
    math_geometry["Math & Geometry"]
    arrays_hashing --> two_pointers
    two_pointers --> stack
    stack --> binary_search
    binary_search --> sliding_window
    sliding_window --> linked_list
    linked_list --> trees
    trees --> tries
    tries --> heap_priority_queue
    heap_priority_queue --> backtracking
    backtracking --> graphs
    graphs --> advanced_graphs
    advanced_graphs --> one_d_dp
    one_d_dp --> two_d_dp
    two_d_dp --> greedy
    greedy --> intervals
    intervals --> bit_manipulation
    bit_manipulation --> math_geometry

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Topic Navigation

Order	Topic	First Patterns	Folder
1	Arrays & Hashing	Frequency Counting, Hash Lookup, Prefix Sum	arrays-hashing/
2	Two Pointers	Opposite Direction Pointers, Same Direction Pointers, Fast And Slow Pointers	two-pointers/
3	Stack	LIFO Simulation, Balanced Delimiters, Monotonic Increasing Stack	stack/
4	Binary Search	Classic Target Search, Lower Bound, Upper Bound	binary-search/
5	Sliding Window	Fixed Window, Variable Window, Frequency Window	sliding-window/
6	Linked List	Dummy Head, Two Pointer Gap, Fast And Slow Pointers	linked-list/
7	Trees	Recursive DFS, Iterative DFS, BFS Level Order	trees/
8	Tries	Prefix Insert And Search, Wildcard Trie DFS, Board Search Trie Pruning	tries/
9	Heap / Priority Queue	Top K Heap, K-way Merge, Two Heaps	heap-priority-queue/
10	Backtracking	Subsets, Combinations, Permutations	backtracking/
11	Graphs	DFS Traversal, BFS Traversal, Connected Components	graphs/
12	Advanced Graphs	Dijkstra, Bellman-Ford, Floyd-Warshall	advanced-graphs/
13	1-D Dynamic Programming	State Definition, Memoization, Tabulation	1d-dp/
14	2-D Dynamic Programming	Grid DP, Two String DP, Knapsack Table	2d-dp/
15	Greedy	Sort And Scan, Greedy With Proof, Interval Greedy	greedy/
16	Intervals	Merge Intervals, Insert Interval, Sweep Line	intervals/
17	Bit Manipulation	XOR Cancellation, Bit Counting, Masks For Sets	bit-manipulation/
18	Math & Geometry	Modulo Arithmetic, GCD And LCM, Prime Sieve	math-geometry/

Beginner Path

Start with arrays, hashing, two pointers, stack, binary search, and sliding window. These topics teach the basic loop invariants used everywhere else. Then move to linked lists, trees, heaps, backtracking, graphs, and dynamic programming.

Beginner rule: do not start Medium practice in a topic until you can explain the Easy solutions without looking at notes.

Interview Sprint Path

When interviews are within 8 weeks, prioritize:

1. Arrays & Hashing
2. Two Pointers
3. Sliding Window
4. Stack
5. Binary Search
6. Trees
7. Graphs
8. Heap / Priority Queue
9. 1-D Dynamic Programming
10. Backtracking

Add Advanced Graphs, 2-D DP, Greedy, Intervals, Bit Manipulation, and Math after the core patterns are stable.

Progress Checklist

• Arrays & Hashing: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Two Pointers: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Stack: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Binary Search: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Sliding Window: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Linked List: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Trees: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Tries: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Heap / Priority Queue: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Backtracking: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Graphs: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Advanced Graphs: README, patterns, cheatsheet, Easy, Medium, selected Hard
• 1-D Dynamic Programming: README, patterns, cheatsheet, Easy, Medium, selected Hard
• 2-D Dynamic Programming: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Greedy: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Intervals: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Bit Manipulation: README, patterns, cheatsheet, Easy, Medium, selected Hard
• Math & Geometry: README, patterns, cheatsheet, Easy, Medium, selected Hard

Repository Maps

• ROADMAP.md: learning order, prerequisites, and outcomes.
• STUDY_PLAN.md: 8-week, 12-week, and 24-week plans.
• INTERVIEW_GUIDE.md: interview communication and execution strategy.
• REPO_INDEX.md: generated index of topics, patterns, and problem counts.

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