Splitwise (Expense Sharing)

Splitwise (Expense Sharing)

Group-expense accounting with integer-exact splits and greedy debt simplification — the most finance-flavored LLD classic.

Requirements

• All money is integer cents. Sum of all balances is always 0 — state this invariant and preserve it in every operation.
• Split remainders deterministically: ⌊amount/k⌋ each, then the first amount mod k users in the participants list pay one extra cent.
• The payer's balance rises by the full amount paid, then falls by their own share if they appear in the participants list.
• simplify is a pure query — running it twice in a row returns the same number.

API to implement

Implement the class Splitwise. The I/O driver is already written in the starter code — it parses the command script below and calls your methods. You only implement the class.

| Command | Returns | Behavior | |---|---|---| | expense <payer> <amount> <participants> | — | payer paid amount (cents) split equally among participants — a comma-separated user list with no spaces (it may or may not include the payer). Integer split: each owes ⌊amount/k⌋; the leftover cents go +1 each to the earliest users in list order. | | balance <user> | long | Net balance of user in cents: positive means the group owes them, negative means they owe. Unknown users are 0. | | settle <fromUser> <toUser> <amount> | — | fromUser hands toUser amount cents in cash, adjusting both balances. | | simplify | int | Without changing any state, return how many transactions the greedy min-cash-flow algorithm needs to settle all debts: repeatedly match the largest creditor with the largest debtor (ties → lexicographically smaller user id) and settle min(credit, debt). |

Input format

The first line contains Q, the number of operations. Each of the next Q lines is one command as listed above. String arguments contain no spaces.

Output format

For each command with a non-void return, print its result on its own line, in order. The driver does this for you — your methods just return values.

This is a design problem: the hidden tests exercise edge cases and interaction sequences, not performance tricks. Model the state cleanly and the tests will pass.

10
expense alice 300 alice,bob,carol
balance alice
balance bob
expense bob 100 alice,carol
balance bob
balance carol
simplify
settle carol alice 50
balance carol
simplify

200
-100
0
-150
1
-100
1

6
expense a 1000 b,c,d
balance a
balance b
balance c
balance d
simplify

1000
-334
-333
-333
3