"Given an array of n integers, return the smallest k numbers, in order."

This simple problem has a few solutions:
- Sorting: O(n log n) time, O(n) space
- Min-heap: O(n + k log n) time, O(n) space
- Max-heap: O(n log k) time, O(k) space
- Quickselect + sorting: O(n + k log k) time, O(n) space