sorting

Merging intervals is a staple that probes the sort-then-sweep idea. The tell is sorting by start and merging in a single pass, O(n log n). Below is the pattern along with the family of problems it opens up.

The fewest meeting rooms you need equals the highest number of meetings active simultaneously. That reframing is the entire interview; two O(n log n) solutions drop out of it, and the tie-handling catches careless candidates.

The naive triple loop runs O(n cubed). Sort first, then fix one element and converge two pointers for O(n squared). The real test is deduplication: returning unique triplets without a set. Here is the clean template.

Merging intervals is the entry point to the whole interval family. The trick is sorting by start so overlaps end up next to each other, then collapsing them in one linear sweep. Here is the pattern and the sweep-line generalization behind it.