Merge Intervals (Scheduling)

Question

## Problem

Merge overlapping time intervals (e.g., busy slots) and find free gaps.

## Example
```
[[1,3],[2,6],[8,10]] -> busy [[1,6],[8,10]]
```

## Constraints
- 1 ≤ n ≤ 10^4

## Approach
Sort, sweep; derive gaps between merged intervals.