USACO 2023 US Open — full contest set
← All past problems · Official 2023 US Open results
The US Open is the final round of the season — 5 hours, three problems per division across Bronze, Silver, Gold, and Platinum. Top scorers across the year (weighted by this round) earn an invitation to the summer training camp where the IOI team is chosen. This page is my landing for all 12 problems from 2023.
Round at a glance
| Field | Value |
|---|---|
| Contest window | March 24–27, 2023 |
| Duration | 5 hours, individual, open-book within the contest window |
| Divisions | Bronze, Silver, Gold, Platinum |
| Problems per division | 3 (12 total) |
| Scoring | Out of 1000 per division; partial credit via subtasks (Silver+) |
| Significance | Final round of the 2022–23 season; top US scorers across all rounds invited to summer camp where the IOI team is selected |
| Allowed languages | C++17, Java, Python 3 (C++ recommended at Gold/Platinum) |
| Recurring theme | "Pareidolia" — three different problems on counting bessie subsequences at Silver / Gold / Platinum |
Divisions
Bronze · strings, grammar, rotations
FEB · Moo Language · Rotate and Shift. Counting valid assignments of an ambiguous string, a greedy sentence builder, and a circular dance that compresses into permutation powers.
Silver · prefix sums, meet-in-the-middle
Milk Sum · Field Day · Pareidolia. Sort + offline queries, SOS / superset DP on C ≤ 18 bitmasks, and the first variant of the bessie-subsequence theme.
Gold · graphs & tricky DP
Custodial Cleanup · Pareidolia · Tree Merging. Key-graph reachability with constructive BFS, weighted-cost subsequence DP, and a constructive tree-isomorphism merge.
Platinum · segment trees & tree math
Pareidolia · Good Bitstrings · Triples of Cows. Segment-tree composition over an automaton, Stern–Brocot / Euclidean-like recursion on bitstrings, and triple-counting on a contracting tree.
How I'm using this set
- Bronze and Silver: real timed 60-min attempts, full implementation, then editorial comparison.
- Gold: read the statement and constraint, brainstorm an approach for 20 min before peeking at the key idea.
- Platinum: study problems — read editorial, then reimplement from memory the next day.
- Compare all three "Pareidolia" problems back-to-back to see how the same surface idea scales from Silver to Platinum.