U USACO Prep

USACO 2018 US Open — the national finals, all four divisions.

The US Open is the season-ending championship: a 5-hour timer per division instead of the usual 4, and the round that determines the four-person USA team for IOI. This page indexes the 2018 edition end-to-end — Bronze through Platinum, with official statement links, key ideas, complexity targets, and runnable C++ references on the per-division pages.

Bronze (all 3) → Silver (all 3) → Gold (all 3) → Platinum (all 3) →
Authoritative source. All problem titles, constraints, and results below are taken from the official US Open 2018 results page on usaco.org: usaco.org/index.php?page=open18results. Each problem links to its canonical statement (statements live at usaco.org/index.php?page=viewproblem2&cpid=…, cpids 831–842).

Round metadata

ContestUSACO 2018 US Open — final round of the 2017–2018 season, national championship
Window4-day window in spring 2018 (single 5-hour personal timer)
Length per division5 hours (US Open format; Dec/Jan/Feb rounds are 4 hours)
Problems per division3
Total problems12 (Bronze 1–3, Silver 1–3, Gold 1–3, Platinum 1–3)
ScoringIOI-style partial credit, 1000 points per problem, 3000 max per division
Allowed languagesC, C++11, Java, Python 2.7, Python 3.6 (C++17 became standard later)
SignificanceSets the four-person USA team for IOI 2018; promotion cutoffs published on the results page.
NotableNo perfect Platinum scores in the pre-college division — the "Train Tracking" interactive problem held everyone short.

The contest at a glance

Bronze

Bronze · 3 problems

1. Team Tic Tac Toe — count single-cow and two-cow team wins on a 3×3 board of letters.

2. Milking Order — earliest position for cow 1 given a hierarchy and fixed-position constraints.

3. Family Tree — classify the relationship of two cows in a mother-child family tree.

Open Bronze write-up →
 Silver

Silver · 3 problems

1. Out of Sorts — count bubble-sort outer passes; equals the max leftward distance any element travels.

2. Lemonade Line — sort cows by patience descending; greedy count of cows who actually join.

3. Multiplayer Moo — largest single-ID and two-ID connected region on a grid (flood fill + per-pair BFS).

Open Silver write-up →
 Gold

Gold · 3 problems

1. Out of Sorts — cocktail-sort version; count passes via per-element bidirectional displacement.

2. Milking Order — maximize prefix of hierarchy constraints; binary search + topological sort.

3. Talent Show — fractional knapsack: maximize Σt / Σw subject to Σw ≥ W (binary search on ratio + DP).

Open Gold write-up →
 Platinum

Platinum · 3 problems

1. Out of Sorts — partition-then-recurse hybrid; total work = Σ over positions of (rounds it crosses a boundary).

2. Train Tracking — interactive sliding-window minimum with only 5,500 notebook slots between car arrivals.

3. Disruption — for each tree edge, find the cheapest extra edge that re-spans the two halves (small-to-large + std::set).

Open Platinum write-up →

How to use this set

  1. Pick your division. Open the full division page and read all three statements before writing any code.
  2. Five hours, not four. US Open gives you the extra hour. Plan for a long P3 push.
  3. P1 first, P2 if time, P3 only if cruising. Open P1s are usually the cheapest points.
  4. Time-box. Don't spend more than ~90 minutes on a problem without a working subtask submission.
  5. Compare to the reference C++. Each problem on the division page has a ~30–50 line solution (Platinum slightly longer). If yours is much longer, ask why.
  6. Verify with the editorial. Official editorials are linked from each problem page on usaco.org.