Resources & references

Curated, low-noise links. Bookmark these; ignore most of the algorithmic-prep TikTok and YouTube ecosystem.

Official

usaco.org — Contest registration, past problems, official editorials, scoring history.
Contest archive — Every problem since the contest started, organized by season and division.
Official resources page — USACO's own list of recommended training materials.

USACO Guide (usaco.guide) — The single best curriculum for USACO. Organized by division, every topic has problems sorted by difficulty with personal-progress tracking.
CP-Algorithms — Encyclopedic reference for algorithms and data structures. Read after you've seen a topic, not as a first introduction.
Codeforces "EDU" courses — Free interactive courses on segment trees, suffix arrays, etc. Quality is excellent.
AtCoder Educational DP Contest — 26 problems that cover every flavor of DP you'll meet at Gold. Indispensable.

Codeforces — Live weekly contests, massive problem archive, real-time rating system. Solve Div 2 A/B/C for Silver/Gold-equivalent practice.
AtCoder — Beautiful, well-tested problems. ABC (Beginner) contests roughly map A–D = Bronze–Silver, E–F = Gold, G+ = Platinum.
Library Checker — Verify your implementations of standard algorithms (segment tree, suffix array, max flow) against tests.
oj.uz — IOI / JOI / CEOI past problems. Roughly Gold to Platinum level.

Competitive Programmer's Handbook — Antti Laaksonen. Free PDF. The single best book to read cover-to-cover at Silver/Gold.
Competitive Programming 4 — Steven & Felix Halim. Comprehensive (two volumes); reference rather than read-front-to-back.
Introduction to Algorithms — Cormen, Leiserson, Rivest, Stein (CLRS). Heavy theory book; useful as a deep reference, not as contest prep.

USACO Guide solutions — Community-written editorials for almost every USACO problem, often with cleaner code than the official ones.
Codeforces blogs — Search for "USACO XXXX YYYY editorial" and you'll find post-mortems from top scorers.
Errichto on YouTube — Clear contest walkthroughs, mostly Codeforces but conceptually transferrable.
William Lin — Former US IOI gold medalist, lots of USACO-specific content.

g++ with -std=c++17 -O2 -Wall -Wextra. On Mac: brew install gcc (Apple's clang works for most things, but Mac's g++ is symlinked to clang by default — for true GCC use the brew version).
VS Code with the C/C++ extension. Set up a build task that compiles and runs sol.cpp < in.txt.
A stopwatch. Honest time tracking on every problem is the single highest-ROI habit.
A notebook (paper or digital) for "patterns I've seen and what they were." Reviewing this monthly is how Gold solvers level up.

Term	What it means
AC	Accepted (all test cases passed)
WA	Wrong Answer
TLE	Time Limit Exceeded
MLE	Memory Limit Exceeded
RE	Runtime Error (segfault, divide-by-zero, out-of-bounds)
OE	Output Limit Exceeded (you printed too much)
CE	Compile Error
DP	Dynamic programming
DSU	Disjoint set union (= Union-Find)
BIT	Binary indexed tree (= Fenwick tree)
LCA	Lowest common ancestor (in a tree)
MST	Minimum spanning tree
SCC	Strongly connected component
HLD	Heavy-light decomposition
CHT	Convex hull trick
FFT/NTT	Fast Fourier / Number-Theoretic Transform