Currency Conversion via Exchange Rate Graph Coding / DSA Interview

You are building a currency exchange system. You receive a list of direct exchange rates, each described as (from_currency, to_currency, rate), meaning 1 unit of from_currency buys `rate` units of to_currency. Design and implement a system that: 1. Ingests the list of exchange rates. 2. Answers queries: given a source currency and a target currency, return the effective conversion rate (or indicate that conversion is not possible). Assume inverse rates are implied: if A→B = r, then B→A = 1/r. Assume every provided rate is positive. If the same directed pair appears multiple times, the last provided rate replaces earlier ones, and the final rate table is internally consistent so any valid path yields the same effective rate. Return 1.0 for a known currency converted to itself, and -1.0 when either currency is unknown or no path exists. Start with any clarifying questions.