Coinbase Mobile Engineer Interview Questions

To improve the efficiency in how fees are applied, Coinbase needs a method to determine the best strategy for applying fees based on transaction patterns using dynamic programming.
Problem statement: You are given a list of integers transactionFees, where each integer represents the fee for a specific transaction. Your goal is to minimize the total fee based on reusing previous fee additions. Return the minimum total fee for any series of transactions.
- Method: minTransactionFee(transactionFees: List[int]) -> int — Returns the minimum total fee possible.Example 1:
Input: transactionFees = [10, 15, 20]
Output: 15
Explanation: The optimal fee by considering all transaction fees at once leads to the lowest possible fee 15.Example 2:
Input: transactionFees = [100, 1, 2, 50, 200]
Output: 3
Explanation: The optimal choices are to skip the high fees and take the next minimal fees that lead to the least total.Constraints:
- 1 <= transactionFees.length <= 10^4
- 1 <= transactionFees[i] <= 10^4

In order to enhance user experience, Coinbase needs a feature to provide real-time updates on user balances by tracking transactions in a sliding window.
Problem statement: Given an array of integers transactions representing the amounts of transactions made by a user over a given period, find the maximum balance within the last k transactions. This will help users quickly understand their recent financial changes.
- Method: maxBalance(transactions: List[int], k: int) -> int — Returns the maximum balance observed within the latest k transactions.Example 1:
Input: transactions = [100, -30, 20, 50, -10], k = 3
Output: 70
Explanation: The last three transactions are [20, 50, -10], leading to a maximum balance of 70.Example 2:
Input: transactions = [200, 400, 600, -500], k = 2
Output: 600
Explanation: The last two transactions are [600, -500], leading to a maximum balance of 600.Constraints:
- 1 <= transactions.length <= 10^5
- -1000 <= transactions[i] <= 1000
- 1 <= k <= transactions.length