The LeetCode Textbook (Java)

A pattern-first, beginner-friendly guide to 100 LeetCode problems from Easy to Medium. Every problem is taught the same way: Intuition -> Pseudocode -> Java -> Complexity -> Dry-run. The goal is not to memorize 100 answers. It is to learn ~16 reusable patterns so any new problem becomes recognizable.

Verified: 100/100 problems compile and pass all tests (679 test cases total). Run javac Solution.java SolutionTest.java && java SolutionTest inside any problem folder.

Full curriculum table: PROBLEM-MAP.md.

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Why this book exists

Most beginners fail LeetCode for one reason: they treat every problem as a brand-new puzzle. Experienced solvers do the opposite -- they map a problem onto a known pattern in seconds, then write code. This book teaches that mapping explicitly.

The structure is borrowed from the proven NeetCode 150 / Blind 75 roadmap, expanded to exactly 100 problems organized into 16 patterns, ordered easiest-first.

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How to read this book

1. Start with 00-how-to-use-this-book.md (10 minutes).
2. Skim 01-patterns-overview.md -- the pattern-recognition cheat sheet. Bookmark it. You will return constantly.
3. If your Java is rusty, read 03-java-crash-course.md.
4. Open patterns/01-arrays-hashing/ and read the pattern's README.md first (it explains when the pattern applies and gives a template).
5. Then do the problems in order inside that folder. For each problem:
   • Read the Intuition section. Try to solve it yourself for 20 minutes first.
   • Answer Checkpoint A (a 2-question concept check, right after Intuition) before reading on.
   • Read the Pseudocode. Make sure you understand the algorithm language-agnostically.
   • Read the Java solution. Compile and run the test.
   • Read the Dry-run to see the algorithm executed on a concrete input.
   • Answer Checkpoint B (trace + edge-case + adapt questions) before moving on.

The rule: never read the Java before you understand the pseudocode. Pseudocode is the bridge between human thinking and machine code. Skipping it is why beginners get stuck memorizing syntax instead of learning patterns. Each pattern's README also ends with a Pattern Mastery Quiz -- treat it as the gate before moving to the next pattern. Quiz answers are collapsible: attempt first, then reveal.

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Pattern Map (the entire book on one page)

#	Pattern	Problems	Difficulty ramp	What you learn
1	Arrays & Hashing	8	Easy	Sets, maps, counting
2	Two Pointers	6	Easy->Med	In-place array ops
3	Sliding Window	6	Easy->Med	Range queries
4	Stack	5	Easy->Med	LIFO, monotonic stack
5	Binary Search	7	Easy->Med	O(log n) on sorted data
6	Linked List	7	Easy->Med	Pointer manipulation
7	Trees	9	Easy->Med	Recursion, BFS/DFS
8	Tries	4	Med	Prefix trees
9	Heap / Priority Queue	5	Easy->Med	Top-K, scheduling
10	Backtracking	6	Med	Permutations, subsets
11	Graphs	9	Easy->Med	BFS, DFS, union-find
12	Greedy	6	Easy->Med	Local optimal -> global
13	Intervals	4	Med	Merge, sweep
14	1-D Dynamic Programming	8	Easy->Med	Memoization, tabulation
15	2-D Dynamic Programming	7	Med	Grid + string DP
16	Bit Manipulation	3	Easy->Med	XOR, masks

Total: 100 problems across 16 patterns.

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Folder layout

leetcode-textbook/
+- README.md                          <- you are here
+- 00-how-to-use-this-book.md
+- 01-patterns-overview.md
+- 02-complexity-cheatsheet.md
+- 03-java-crash-course.md
+- patterns/
|   +- 01-arrays-hashing/
|   |   +- README.md                  <- pattern intro + template
|   |   +- 217-contains-duplicate/
|   |   |   +- README.md              <- problem + pseudocode + walkthrough
|   |   |   +- Solution.java
|   |   |   +- SolutionTest.java
|   |   +- 242-valid-anagram/
|   |   +- ...
|   +- 02-two-pointers/
|   +- ...
+- docs/
    +- changelog-2026-07-05.md

Every problem folder is self-contained: read README.md, open Solution.java, run SolutionTest.java to verify.

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How to run a problem's tests

Each SolutionTest.java uses a plain main method (no JUnit dependency needed). From inside any problem folder:

javac Solution.java SolutionTest.java && java SolutionTest

If you see All tests passed. the solution is correct on the included cases. Java 11+ is required (javac -version to check).

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Difficulty philosophy

This book goes Easy -> Medium. It deliberately excludes Hard problems because:

• 90% of interview questions are Easy/Medium.
• Patterns transfer upward: a Hard problem is usually a Medium pattern with an extra constraint.
• Mastering 100 Easy/Medium problems beats skimming 30 Hard ones.

Once you finish this book, the standard next step is the NeetCode 150 Hard problems or LeetCode 75.

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Credits

Curriculum structure adapted from NeetCode (neetcode.io) and the Blind 75 list. All solutions, pseudocode, and teaching notes written fresh for this book.

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Built July 2026.