Customer Revenue and Referral Tracking

Design a data structure that tracks customer revenue and referral credit. Implement the CustomerRevenueTracker class:

• CustomerRevenueTracker() Initializes the tracker.
• int insertNewCustomer(int revenue) Inserts a new customer with the next available customer ID, starting from 0. The new customer begins with total revenue revenue.
• int insertNewCustomerWithReferral(int revenue, int referrerId) Inserts a new customer with the next available customer ID. The new customer begins with total revenue revenue, and the referrer's total revenue also increases by revenue.
• int[] getLowestK(int k, int minTotalRevenue) Returns up to k customer IDs whose total revenue is at least minTotalRevenue, choosing the customers with the smallest total revenue first. In this practice version, revenue values are integers. Order the returned customer IDs by increasing (totalRevenue, customerId). This deterministic ordering is only for the practice runner; the original interview prompt may describe the result as a set. The original interview prompt may also present the referral form as an overloaded insertNewCustomer(revenue, referrerId). This practice runner exposes it as insertNewCustomerWithReferral(...) so all supported languages can be evaluated consistently.

Customer 0 starts at 10, then gains 30 from customer 1, so total revenue becomes 40. Customer 1 starts at 30, then gains 50 from customer 2, so total revenue becomes 80. Customer 2 has total revenue 50. The qualifying customers for threshold 45 are (2, 50) and (1, 80), so the lowest one is [2] and the lowest two are [2, 1].

Before the referral, customers 0 and 1 both have total revenue 20, so the smaller ID comes first. After inserting customer 2 with referral to 0, totals become 25, 20, and 5, so the qualifying customers are [1, 0].

At most 10^4 total calls will be made to insertNewCustomer, insertNewCustomerWithReferral, and getLowestK.