Stripe Mobile Engineer Interview Questions

Stripe operates globally, dealing with multiple currencies, making effective currency conversion essential to minimize financial loss during conversions.
Given a list of currency rates where each element represents the rate to convert from one currency to another, return the maximum amount you can convert from one currency to another using a greedy algorithm for optimal conversion.
- Input: List<double> rates
- Output: double — the maximum amount achievable after conversion.Example 1:
Input: [0.74, 1.5, 0.6]
Output: 1.11
Explanation: The best conversion path is from 0.74 to 1.5.Example 2:
Input: [1.0, 0.9, 1.2]
Output: 1.2
Explanation: No losses occur; simply pick the best rate.Constraints:
- 1 <= rates.length <= 100
- 0 < rates[i] <= 10^2

In the context of Stripe's payment processing system, it’s crucial to understand which payment methods are failing consistently over time. This information can help in identifying issues and improving the payment experience.
Given a list of integers representing the payment failures over the last n hours, implement a function that determines the maximum number of failures within any continuous window of k hours.
- Input: List[int] failures, int k
- Output: int — the maximum number of failures in any sliding window of size k.Example 1:
Input: [1, 2, 1, 3, 2, 1], 3
Output: 5
Explanation: The sliding window [1, 3, 2] has a sum of 5 failures.Example 2:
Input: [2, 3, 4, 5], 2
Output: 7
Explanation: The sliding window [3, 4] has a sum of 7 failures.Constraints:
- 1 <= failures.length <= 10^5
- 1 <= failures[i] <= 10^4
- 1 <= k <= failures.length

Offline capabilities are essential for Stripe's mobile users, enabling transaction management without losing data when connectivity is limited. Design an Offline-First Sync Engine that ensures transactions are synchronized efficiently when online.
You should implement a class OfflineSyncEngine with methods to add, sync, and retrieve transactions.
- Method 1: addTransaction(Transaction tx) - adds a transaction locally.
- Method 2: syncTransactions() - syncs all locally added transactions to the server when online.
- Method 3: getTransactions() -> List<Transaction> - retrieves the list of transactions.Example 1:
Input: addTransaction(new Transaction(...))
Output: None
Input: syncTransactions()
Output: None
Input: getTransactions()
Output: List<Transaction>Constraints:
- The engine can handle up to 1000 transactions.

As part of Stripe’s financial services, representing fluctuating currency exchange rates in a tree structure can enable efficient querying and updates. This task involves creating an appropriate tree design to serve those needs.
You are to implement a class CurrencyExchangeTree with methods to add rates, remove rates, and retrieve the rate between two currencies efficiently.
- Method 1: addRate(String from, String to, double rate) - adds a new exchange rate.
- Method 2: removeRate(String from, String to) - removes an exchange rate.
- Method 3: getRate(String from, String to) -> double - Retrieves the exchange rate between two currencies.Example 1:
Input: addRate('USD', 'EUR', 0.84)
Output: None
Input: getRate('USD', 'EUR')
Output: 0.84Constraints:
- The tree can have at most 100 unique currency nodes.