Net Revenue Per Account

Problem Context

You are building a billing analytics system for a SaaS platform. The system receives a stream of billing events, where each event is either a charge (money collected from an account) or a refund (money returned to an account). Your task is to process these events and compute the net revenue for each account.

Input Format

You are given a list of billing events. Each event is represented as a tuple/list of three elements:

• account_id (string): unique identifier for the account
• event_type (string): either "charge" or "refund"
• amount (integer): the dollar amount of the event (always positive)

Return a dictionary mapping each account_id to its net revenue, where:

• A charge increases the net revenue
• A refund decreases the net revenue

The result should include all accounts that appear in the events. Accounts with a net revenue of zero should still be included.

Output Format

Return a dictionary { account_id: net_revenue } sorted by account_id alphabetically (or return as a regular dict — order does not matter for correctness).

Examples

Example 1

Input:

events = [
  ["acct_A", "charge", 100],
  ["acct_B", "charge", 200],
  ["acct_A", "refund", 30]
]

Output: {"acct_A": 70, "acct_B": 200}

Explanation: acct_A had a charge of 100 and a refund of 30, so net = 70. acct_B had only a charge of 200.

Example 2

Input:

events = [
  ["acct_X", "charge", 500],
  ["acct_X", "refund", 500]
]

Output: {"acct_X": 0}

Explanation: The charge and refund cancel out, resulting in a net revenue of 0. The account still appears in the output.

Example 3

Input:

events = []

Output: {}

Explanation: No events means no accounts and an empty result.

Approach Hints

• Consider using a hash map (dictionary) to accumulate net revenue per account in a single pass.
• Think about how to handle accounts that appear only in refunds — their net revenue may be negative.
• The overall time complexity should be O(n) where n is the number of events.