Currency CLI: Build a Command-Line Currency Converter with Cached Exchange Rates

Question

## Round 1 - Coding

## Problem

Implement a command-line currency converter. The CLI accepts commands to set exchange rates, convert amounts, and query conversion chains (convert through an intermediate currency if a direct rate isn't available).

```python
class CurrencyCLI:
    def set_rate(self, from_curr: str, to_curr: str, rate: float) -> None:
        ...
    def convert(self, amount: float, from_curr: str, to_curr: str) -> float:
        # returns -1.0 if conversion is impossible
        ...
    def best_rate(self, from_curr: str, to_curr: str) -> float:
        # returns the effective rate via the best available path
        ...
```

## Example

```
cli = CurrencyCLI()
cli.set_rate("USD", "EUR", 0.92)
cli.set_rate("EUR", "GBP", 0.86)
cli.convert(100, "USD", "EUR")  -> 92.0
cli.convert(100, "USD", "GBP")  -> 79.12   # USD->EUR->GBP
cli.convert(100, "USD", "JPY")  -> -1.0    # no path
cli.best_rate("USD", "GBP")    -> 0.7912
```

## Follow-ups

1. Rates are bidirectional — if USD->EUR is 0.92, what is EUR->USD? How do you store both?
2. How do you find the path that maximizes the converted amount (best rate) rather than any valid path?
3. How do you detect arbitrage opportunities (a cycle where converting back yields more than you started)?
4. How would you cache rates with a TTL so stale rates expire after 60 seconds?

Currency CLI: Build a Command-Line Currency Converter with Cached Exchange Rates

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