Implement a Trie for UPI ID Autocomplete

Question

## Problem

Implement a **Trie (prefix tree)** that supports UPI ID autocomplete.

## Required operations
- `insert(upiId)` — add a UPI ID (e.g., "shivam@paytm")
- `search(upiId)` → true/false exact match
- `startsWith(prefix)` → true/false prefix exists
- `autocomplete(prefix, topK)` → return up to K UPI IDs matching the prefix, sorted by frequency

## Example
```
insert("alice@paytm") x3, insert("alex@paytm") x1, insert("bob@upi") x2
autocomplete("al", 2) → ["alice@paytm", "alex@paytm"]
```

## Implementation notes
- Each Trie node stores: children map, isEnd flag, frequency counter
- `autocomplete` does DFS from the prefix node, collects all words, returns top-K by frequency

## Extension
How would you make this thread-safe for concurrent inserts and searches?