Splitwise — Simple

Problem statement (interviewer prompt)

Design Splitwise: groups of friends log shared expenses (equal / unequal / by share / by percentage), and the app shows who-owes-whom balances. Add a "simplify debts" feature that reduces the number of payments in a group to settle up.

flowchart LR
  U([User])
  EXP[Expense Service]
  GRP[Group / Friends]
  BAL[Balance Service]
  SETT[Settlement / Simplify]
  U --> EXP --> BAL
  GRP --- EXP
  BAL --> SETT

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class U client;
    class EXP,GRP,BAL,SETT service;