Find k customers with least revenue

Q: Find k customers with least revenue

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Question

Problem

You are given a list of purchase events. Each event contains:

customer_id (string or int)
amount (integer, may be 0; assume non-negative unless stated otherwise)

Define a customer's revenue as the sum of amount across all events for that customer.

Task

Return the k customers with the smallest total revenue.

Requirements

If fewer than k customers exist, return all customers.
If multiple customers tie on total revenue, you may return them in any order (or specify a deterministic tie-breaker such as customer_id ascending).

Input/Output (suggested)

Input: events: List[(customer_id, amount)] , k: int
Output: List[customer_id] (the k least-revenue customers)

Constraints (assumptions)

n = len(events) can be large.
Number of unique customers m can be up to n .