Requirements

• Users need to register on Flipkart to use this wallet.
• The user can load money into his wallet via various sources (Credit Card, Debit Card, UPI etc).
  • Minimum amount of load money should be greater than 0.
  • Assumption - No need for Integration from sources, can be implemented just success acknowledged.
  • When user loads money, current system timestamp (monotonically increasing) is passed along and that is what is used for sorting transactions.
• The user can send money to other users from their wallet.
  • The minimum amount of transaction will be always greater the 0
  • The user must have a sufficient balance in his wallet while doing the transaction.
  • When user sends money, current system timestamp (monotonically increasing) is passed along and that is what is used for sorting transactions.
• The user can fetch their wallet balance at any point of time, this should consider both credit & debit type of transactions.
• The user can get transaction history based on
  • Sort their all transactions history
    • Based on Amount ("amount")
    • Based on Transaction DateTime ("time") using the passed timestamps
  • Filter transactions history
    • Based on Send or Receive Amount ("send/receive")
  • Assumption: Consider single filter & sorted field

Assumptions & Clarifications

• All userIds passed to methods are Flipkart-registered users. A non-registered userId is treated as invalid.
• Every successful money load or send operation creates transaction records.
• The caller provides a monotonically increasing timestamp on each loadMoney and sendMoney. This passed timestamp is stored in the transaction and used for sorting when sortBy="time".
• If two transactions have the same timestamp, preserve insertion order (stable sort) while sorting by time.
• A sendMoney operation creates two records when successful (both share the same passed timestamp):
  • Sender: SEND (debit)
  • Receiver: RECEIVE (credit)
• Wallet balance is computed as: sum(credits) - sum(debits). Wallet balance must be fetched using a separate method getBalance.
• Sorting uses exactly one field per call:
  • sortBy="time": ascending by passed timestamp
  • sortBy="amount": highest to lowest (descending amount)
• Filtering uses exactly one filter per call:
  • filterBy="send": only SEND transactions
  • filterBy="receive": only RECEIVE transactions
  • filterBy="all": LOAD, SEND, and RECEIVE

Data Encoding

• A transaction is encoded as a single string: time=<timestamp>|type=<LOAD|SEND|RECEIVE>|counterparty=<value>|amount=<a>
• counterparty is:
  • For LOAD: the load source (e.g., UPI, CreditCard)
  • For SEND: the receiver userId
  • For RECEIVE: the sender userId
• getTransactionHistory returns only transaction strings (no balance line).

Class & Method Signatures

Constructor

FlipkartWallet(List<String> registeredUserIds)

• Initializes the system with the set of Flipkart-registered users.

1) Load Money

boolean loadMoney(String userId, long amount, String source, long timestamp)

• 0 < amount < 10^9
• timestamp > 0
• Returns true if load is successful; otherwise false (invalid user or invalid amount).
• Creates one LOAD (credit) transaction on success with time=timestamp.

2) Send Money

boolean sendMoney(String fromUserId, String toUserId, long amount, long timestamp)

• 0 < amount < 10^9
• timestamp > 0
• fromUserId and toUserId must be registered users
• fromUserId must have sufficient balance
• Returns true if send is successful; otherwise false.
• Creates two transactions on success (both with time=timestamp):
  • Sender: SEND (debit)
  • Receiver: RECEIVE (credit)

3) Get Balance

long getBalance(String userId)

• Returns current wallet balance for userId considering both credit & debit transactions.
• If userId is invalid (not registered), return -1.

4) Get Transaction History

List<String> getTransactionHistory(String userId, String sortBy, String filterBy)

• sortBy is one of: "amount", "time"
• filterBy is one of: "send", "receive", "all"
• Output format:
  • Returns transaction strings after applying exactly one filter and one sort field.
  • For a valid user, it may return an empty list if there are no matching transactions.
  • If userId is invalid (not registered), return an empty list.

Constraints

• 1 ≤ registeredUserIds.size()
• amount and timestamp fits in signed 64-bit integer.
• loadMoney and sendMoney with amount <= 0 must fail
• sendMoney must fail if balance is insufficient
• getBalance returns -1 for invalid user
• getTransactionHistory returns an empty list for invalid user
• In-memory only; no external DB or source integrations

Examples

Example 1: Load money, send money, fetch balance, and read history (time sort uses passed timestamps)

Example 2: Sort by amount

Example 3: Filter send/receive with time sort (timestamps drive order)