U USACO Prep

USACO 2015 February — the whole contest, three divisions.

February 2015 ran before Platinum existed (Platinum debuted the next season, December 2015). So this round is 3 divisions × 3 problems = 9 problems total — Bronze, Silver, Gold — with the official statement link, the key idea, complexity target, and a runnable C++ reference for each problem on the per-division pages.

Bronze (all 3) → Silver (all 3) → Gold (all 3) → Platinum (n/a — see note) →
Authoritative source. All problem titles and constraints below are taken from the official February 2015 results page on usaco.org: usaco.org/index.php?page=feb15results. Each problem links to its official statement (statements live at usaco.org/index.php?page=viewproblem2&cpid=…, cpids 526–534).
Platinum heads-up. The Platinum division did not exist in February 2015. USACO introduced Platinum in the following season (December 2015). The Platinum page in this set documents that absence and points to the December 2015 round as the first Platinum contest.

Round metadata

ContestUSACO 2015 February
WindowRoughly Feb 13–16, 2015 (4-day window, single 4-hour personal timer)
Length per division4 hours (Dec/Jan/Feb format; US Open is the 5-hour round)
Problems per division3
Total problems9 (Bronze 1–3, Silver 1–3, Gold 1–3) — no Platinum yet
ScoringIOI-style partial credit, 1000 points per problem, 3000 max per division
Allowed languagesC, C++, Java, Pascal, Python (C++ default)
Promotion cutoffsSet per-contest by USACO; check the results page for exact thresholds.

The contest at a glance

Bronze

Bronze · 3 problems

1. Censoring — repeatedly delete the first occurrence of T from S; T is short so a naive scan works.

2. COW — count "COW" subsequences in a C/O/W string; classic 3-state running count.

3. Cow Hopscotch (Bronze) — count strictly down-right paths through differently-coloured cells on a small grid (R, C ≤ 15).

Open Bronze write-up →
 Silver

Silver · 3 problems

1. Censoring — same statement as Bronze but S up to 106: KMP + stack.

2. Cow Hopscotch (Silver) — R, C ≤ 100, K colours; O(R²C²) DP fits.

3. Superbull — single-elimination tournament, score = XOR of IDs; maximum spanning tree on XOR-weighted complete graph.

Open Silver write-up →
 Gold

Gold · 3 problems

1. Cow Hopscotch (Gold) — R, C ≤ 750; O(R²C²) is dead. Per-colour 2D prefix sums + total-prefix subtraction → O(RC log).

2. Censoring — N censored words, total length 105: Aho–Corasick + stack of states.

3. Fencing the Herd — online dynamic convex-hull queries: do all points lie strictly to one side of Ax+By=C? CDQ divide-and-conquer on additions + line queries.

Open Gold write-up →
 Platinum

Platinum · did not exist yet

USACO had only three divisions in the 2014–15 season. Platinum was added one season later, in December 2015. The Platinum page in this set explains that and links to the first real Platinum round.

If you want Platinum-flavoured difficulty from this exact problem set, the Gold problems (especially Hopscotch Gold and Fencing the Herd) are firmly in modern-Platinum territory anyway.

Open Platinum note →

How to use this set

  1. Pick your division. Open the full division page and read all three statements before writing any code.
  2. Solve P1 first. February P1s here are the cheapest points — Censoring Bronze is essentially "naive scan with a stack", COW is one pass.
  3. Time-box. 4 hours total. Don't spend more than ~90 minutes on a single problem without a working subtask submission.
  4. Compare to the reference C++. Each problem on the division page has a ~30–50 line reference solution. If yours is much longer, ask why.
  5. Verify with the editorial. Official editorials are linked from each problem page on usaco.org.