Design an Expense Splitter (Splitwise Clone)

Problem

Design an expense splitting application where groups of friends can add expenses and track who owes whom.

Requirements

• addExpense(paidBy, amount, splitAmong[]) — split equally
• getBalance(userId) — net amount owed/owing
• settle(from, to, amount) — record a settlement
• minimizeTransactions(groupId) — return minimum transfers to settle all debts

Classes expected

User, Group, Expense, ExpenseSplit, ExpenseManager

Key design decisions

• Store individual balances as a net map (not per-expense)
• minimizeTransactions — reduce to a min-cost flow problem; greedy creditor/debtor matching works for equal splits
• Thread safety for concurrent expense additions

Extension

Support unequal splits (by percentage or exact amount).