This set mixes the whole unit without saying which pattern each problem needs. Before you code, decide:
- Is the data sorted, or could sorting it make the problem easy? That points to binary search or two pointers.
- Do you need to find something instantly as each item arrives? That's a dictionary or a set.
- Is the question about a run of consecutive items? That's a window.
- Are the items ranges with a start and an end? Sort by start and sweep.
Several hidden checks use thousands of items, so an answer that's right but quadratic runs out of time. When that happens, the message tells you, and the unit's lessons show the faster pattern.