usaco.org/index.php?page=viewproblem2&cpid=…, cpids 507–516).
Round metadata
| Contest | USACO 2015 January |
|---|---|
| Window | January 2015, 4-day window, single 4-hour personal timer |
| Length per division | 4 hours (Dec/Jan/Feb format; US Open is the 5-hour round) |
| Divisions that existed | Bronze, Silver, Gold (Platinum was added in December 2015) |
| Problems | Bronze had 4, Silver had 3, Gold had 3 — 10 problems total |
| Scoring | IOI-style partial credit, 1000 points per problem |
| Allowed languages | C, C++, C++11, Java, Pascal, Python 2.7, Python 3 (C++/C++11 dominant) |
| Participation | 2,084 participants from 72 countries; 4,971 graded submissions |
| Promotion cutoffs | 600/1000 from Bronze→Silver and Silver→Gold for this contest |
| Grading-server change | Mid-contest, USACO swapped grading servers (the new pool ran at half the speed of the originals); all time limits were doubled and submissions re-graded. |
The contest at a glance
Bronze · 4 problems
1. Cow Routing — minimum cost using a single route to travel from A to B over N airline routes.
2. Cow Routing II — same setup but allow up to two routes; try every pair of board/disembark cities on each pair of routes.
3. It's All About the Base — recover two bases X, Y in [10, 15000] such that two 3-digit numbers represent the same N.
4. Meeting Time — DAG (N ≤ 16) with two edge weights per edge; find smallest T such that Bessie and Elsie can both reach N in exactly T.
Silver · 3 problems
1. Stampede — N cows on horizontal segments moving right at different speeds; sweep by y to count cows that are first-visible to a +y ray from the origin at some moment.
2. Cow Routing — Silver version: any number of routes (pay per use), find min cost + tie-break by min flight count. Modified Dijkstra over (cost, flights).
3. Meeting Time — same DAG as Bronze P4 but with N ≤ 100; use 2-D reachability (per-vertex set of (Bessie-time, Elsie-time)) and intersect at N.
Gold · 3 problems
1. Cow Rectangles — N ≤ 500 points labelled H/G in the plane; max H count enclosed by an axis-aligned rectangle containing zero G's, then min area among such rectangles.
2. Moovie Mooving — N ≤ 20 movies with showtimes, cover [0, L] continuously; bitmask DP on which movies used, value = latest end time.
3. Grass Cownoisseur — directed graph, cycle through 1 allowed to reverse one edge; SCC-condense to a DAG and run two longest-path DPs.
Platinum · did not exist yet
Platinum was introduced eleven months later, in December 2015. The placeholder page documents that and links to the first three real Platinum problems instead.
How to use this set
- Pick your division. Open the full division page and read every statement before writing code.
- Bronze runs four problems. Budget roughly 60 minutes each rather than the usual 80 — and skim P4 first to spot the N ≤ 16 hint.
- Silver P2 reuses Bronze P1's input format. If you're solving both, lift your input parser straight across.
- Gold P3 is the SCC problem of the season. If you've never coded Tarjan or Kosaraju, do this one with the code in front of you the first time.
- Verify with the editorial. Each problem page on usaco.org links the official solution write-up and test data.