Hard problems that push your skills:
Create Maximum Number: - Multi-step greedy with merging
Patching Array: - Range extension with doubling insight
IPO: - Two-constraint optimization with heaps
Minimum Cost to Hire K Workers: - Ratio-based selection
Candy: - Two-pass greedy with constraints from both sides These require combining multiple techniques and careful correctness proofs. These problems demand multi-step reasoning: IPO requires sorting by capital, then a heap for profit selection. Candy needs two passes to satisfy constraints from both directions.