Implement a Prefix Search (Trie)

Question

## Problem

Design a dictionary supporting `insert(word)` and `startsWith(prefix)` for autocomplete.

## Example
```
insert("car"); startsWith("ca") -> true; startsWith("cb") -> false
```

## Constraints
- Up to 10^5 words

## Approach
Trie with children map; discuss memory vs a sorted-array binary search alternative.