4 subtopics · 15 problems
Combinatorics & Counting
Counting, permutations, combinations, inclusion-exclusion, and constructive algorithms.
01Counting & nCk
Combinations, Pascal's triangle, stars and bars.
2 medium2 hard
| Problem | Difficulty | Link |
|---|---|---|
| 1862DIce Cream Balls | medium | Solve |
| 894BRalph And His Magic Field | medium | Solve |
| 1900DSmall GCD | hard | Solve |
| 1837EPlayoff Fixing | hard | Solve |
02Inclusion-Exclusion Principle
Count by adding and subtracting overlapping sets.
3 hard
| Problem | Difficulty | Link |
|---|---|---|
| 451EDevu and Flowers | hard | Solve |
| 1295DSame GCDs | hard | Solve |
| 997CSky Full of Stars | hard | Solve |
03Constructive Algorithms
Build a valid answer rather than optimizing one.
1 easy3 medium1 hard
| Problem | Difficulty | Link |
|---|---|---|
| 1921BArranging Cats | easy | Solve |
| 1829CMr. Perfectly Fine | medium | Solve |
| 1360GA/B | medium | Solve |
| 1515CPhoenix and Towers | medium | Solve |
| 1904DSet To Max | hard | Solve |
04Pigeonhole Principle
If n+1 items go into n boxes, at least one box has ≥ 2 items.
2 medium1 hard
| Problem | Difficulty | Link |
|---|---|---|
| 1516BAGAGA XOOORRR | medium | Solve |
| 1294CProduct of Three Numbers | medium | Solve |
| 735DTaxes | hard | Solve |
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