Part 1 · 23 topics · free

Part 1 — The Core DSA & Algorithm Cheatsheet

Built by analysing all 150 questions first, then extracting the union of every structure, pattern and Java mechanic they require — and nothing they don't. Written to be read cold.

Derived by analyzing all 150 NeetCode questions and taking the union of their prerequisites. If a technique isn't needed by at least one of the 150, it isn't here. If a question needs it, it is.

These files assume no prior knowledge of the topic. Each one explains what the structure or technique is and why it exists before showing code, and every non-obvious algorithm is traced step by step on a concrete example.

Read this first

The quick reference

The vocabulary used throughout, the decision tree that turns a problem shape into a pattern, the ten mechanics that appear most across the 150, and the five arguments worth memorising.

Open the reference →

4sheets

Foundations

Read these first and carefully — everything else assumes them.

01 Complexity & Problem Triage What Big-O means, constraint → approach, how to triage a problem 12 min 02 Java Collections Toolkit What each structure is, what it costs, when to pick it; iteration & utility methods 33 min 03 Java Syntax Mechanics Char math, boxing traps, overflow, arrays, recursion 11 min 04 Ordering: Comparable, Comparator & Sorting Natural vs custom order, multi-key sorting, sorted collections of custom objects, sorting maps 18 min 05 equals, hashCode & Using Objects as Keys Why objects fail as HashMap keys, the contract, what to use instead of int[] 8 min

Patterns

One file per technique family. Read alongside the matching section.

06 Arrays & Hashing Trading space for a removed loop; key design 15 min 07 Two Pointers Using order to eliminate candidates 11 min 08 Sliding Window Incremental summaries over a moving range 14 min 09 Stack & Monotonic Stack Deferred resolution; next-greater problems 12 min 10 Binary Search Finding the boundary of a monotonic predicate 14 min 11 Linked List Dummy heads, fast/slow pointers, reversal, LRU 15 min 12 Trees & Binary Search Trees What each node needs from its children 15 min 13 Tries (Prefix Trees) Prefix structure and search pruning 8 min 14 Heaps / Priority Queues Cheap access to extremes; top-k, k-way merge, median 12 min 15 Backtracking Choose / explore / un-choose; dedup and pruning 14 min 16 Graphs Recognizing graphs; DFS vs BFS; topological sort 13 min 17 Union-Find (Disjoint Set Union) Connectivity as edges arrive 7 min 18 Advanced Graphs Dijkstra, Bellman-Ford, MST, Eulerian paths 10 min 19 Dynamic Programming Recursion plus remembering; the six recurrences 19 min 20 Greedy When a local choice is provably safe 11 min 21 Intervals Sort by start vs by end; sweep line 9 min 22 Bit Manipulation Operators, XOR properties, the eight tricks 11 min 23 Math & Geometry Matrix transforms, fast power, overflow-safe math 12 min