The twelve problems
Each card below summarises one division and links to my full write-up for that division. Problem titles are taken from the official results page; paraphrases are my own.
Bronze division
1. Leaders — count pairs of cows that can each lead the other's breed under a window rule.
2. Air Cownditioning II — minimum air conditioners (each covers a range, with a power budget) to cool every stall.
3. Moo Operations — minimum edits to turn a string into a concatenation of MOOs.
Silver division
1. Air Cownditioning II — same setup as Bronze, larger N — choose a subset of ACs minimising total cost while covering every stall's demand.
2. Spelling Bee — count strings that can be reduced to a target via stack-style "double consecutive" rules.
3. Find and Replace — minimum global letter-replace operations to turn S into T, treating "rename all a→b" as one move.
Gold division
1. Find and Replace — Silver's puzzle with bigger N and tougher cycle/cascade reasoning on the letter graph.
2. Mana Collection — collect mana on a graph where pools regenerate over time; queries ask the maximum mana achievable ending at a given node.
3. Tractor Paths — shortest reachability and counting paths on intervals that overlap on a number line.
Platinum division
1. Tractor Paths — Gold's interval-graph problem extended with edge-count queries between specific intervals.
2. Mana Collection — full version: arbitrary start, query (start, end-time) pairs over a graph with regenerating pools.
3. Subset Sort — count permutations sortable by a restricted set of swaps / segment operations.
How I'm reading this contest
- Air Cownditioning II straddles Bronze and Silver — Bronze gives small N (brute force), Silver wants a real optimisation.
- Find and Replace straddles Silver and Gold — the letter-mapping graph is the central object, and Gold's larger N forces you to actually reason about cycles vs trees.
- Tractor Paths and Mana Collection both appear at Gold and Platinum. Gold often asks one query type; Platinum opens it up to arbitrary queries with stricter time limits.