Compute account balances with rejection and overdraft

You are given a list of transaction records as strings. Each record has the format:

account_id,timestamp,currency,amount

• account_id : string identifier
• timestamp : integer (not guaranteed sorted)
• currency : string like USD , EUR
• amount : integer (can be positive for credit or negative for debit)

Maintain balances per (account_id, currency).

Part 1 — Final balances

Process all transactions and return the final balance for each (account_id, currency).

• Exclude entries whose final balance is exactly 0 .
• Output can be in any order (unless you choose to sort).

Part 2 — Reject transactions that would go negative

Now process transactions in increasing timestamp order (tie-breaker can be input order).

A transaction is rejected if applying it would make that account’s balance for that currency fall below 0.

• Rejected transactions have no effect.
• Return final non-zero balances as in Part 1.

Part 3 — Use a platform account for insufficient funds

Add support for a special “platform” account that can cover shortfalls. You are given platform_account_id as a parameter.

Processing is still in timestamp order. For a debit transaction (negative amount) on account A in currency C:

1. If A has enough balance in C , apply it normally.
2. Otherwise, attempt to cover the shortfall using the platform account’s balance in the same currency C :
   • Deduct all remaining funds from A (bringing it to 0 ).
   • Deduct the remaining required amount from the platform account.
3. If the platform account also lacks sufficient funds in C , reject the transaction (no balances change).

Credits (positive amount) always apply.

Return final non-zero balances for all accounts (including the platform account if non-zero).

Constraints (you may assume)

• Up to ~10^5 transactions
• timestamp fits in 64-bit integer
• amount fits in 64-bit signed integer