U USACO Prep

USACO 2015 US Open — the whole contest, every division that ran.

The 2015 US Open was a 5-hour finals round across three divisions: Bronze (4 problems), Silver (3), and Gold (3) — ten problems total. Platinum did not exist yet; it was introduced for the 2015–16 season starting December 2015. Each per-division page below has statement paraphrases, key ideas, complexity targets, and a runnable C++ reference.

Bronze (all 4) → Silver (all 3) → Gold (all 3) → Platinum (n/a) →
Authoritative source. All titles, cpids, and constraints below come from the official results page on usaco.org: usaco.org/index.php?page=open15results. Problem statements live at usaco.org/index.php?page=viewproblem2&cpid=…, cpids 545–554.

Round metadata

ContestUSACO 2015 US Open (finals round of the 2014–15 season)
WindowRoughly late March 2015, 4-day window with a personal 5-hour timer
Length per division5 hours (US Open is the long round; Dec/Jan/Feb are 4 hours)
Divisions that ranBronze, Silver, Gold — Platinum was introduced the following December 2015
Problems10 total: Bronze 1–4, Silver 1–3, Gold 1–3
ScoringIOI-style partial credit, 1000 points per problem
Allowed languagesC, C++, Java, Pascal, Python
Promotion cutoffsSet per-contest by USACO; check the results page for thresholds.

The contest at a glance

Bronze

Bronze · 4 problems

1. Moocryption — choose a substitution cipher (no fixed points) maximizing count of "MOO" lines in a grid.

2. Bessie Gets Even — count variable assignments making (B+E+S+S+I+E)(G+O+E+S)(M+O+O) even; parity DP.

3. Trapped in the Haybales (Bronze) — total road length from which Bessie cannot escape past either end bale (simulation).

4. Palindromic Paths (Bronze) — count distinct palindromic strings on D/R paths through an N×N letter grid, N ≤ 18 (meet in the middle + set).

Open Bronze write-up →
 Silver

Silver · 3 problems

1. Bessie Goes Moo — count assignments making the (B+E+S+S+I+E)(G+O+E+S)(M+O+O) product divisible by 7 using mod-7 residue counts.

2. Trapped in the Haybales (Silver) — minimum size to add to one bale so Bessie can't escape; simulate both directions.

3. Bessie's Birthday Buffet — eat patches with strictly increasing quality, paying E per step traveled; DP over quality-sorted patches.

Open Silver write-up →
 Gold

Gold · 3 problems

1. Googol — interactive: deduce N (up to 10100) in a left-heavy balanced binary tree via ≤ 70 000 queries; big-int + path probing.

2. Palindromic Paths (Gold) — count D/R palindrome paths in an N×N grid mod 109+7, N ≤ 500; double-front DP from both corners.

3. Trapped in the Haybales (Gold) — same setup as Bronze/Silver, but N ≤ 105 and positions/sizes up to 109; stack/segment-tree simulation.

Open Gold write-up →
 Platinum

Platinum · did not exist yet

The Platinum division was introduced for the 2015–16 season, starting with December 2015. There is no Platinum tier for the 2015 US Open.

Open the page for the receipts, plus pointers to the first real Platinum contest (Dec 2015) if you want a fourth-tier 2015-vintage problem set.

Open Platinum note →

How to use this set

  1. Pick your division. Open the full division page and read every statement before writing code.
  2. Solve P1 first. US Open Bronze P1 is usually the cheapest 1000 points; Silver/Gold P1 is roughly Silver-easy / Gold-easy.
  3. Time-box. 5 hours total, so ~75 minutes per problem in Bronze and ~100 minutes in Silver/Gold.
  4. Compare to the reference C++. If yours is dramatically longer than the ~30–50 line reference, you're probably solving a harder version of the problem.
  5. Verify with the editorial. Official editorials are linked from each problem page on usaco.org.