Banking System Operations

PayPal's "Banking System Operations" interview question focuses on designing an efficient system to process a sequence of banking operations (deposits, withdrawals, transfers) while supporting queries for top-k balances or transactions, leveraging hash tables for fast lookups and heaps for priority-based retrieval.[1]

Problem Statement

Design a BankingSystem class that handles operations on multiple bank accounts. It must support:

• Adding accounts with initial balances.
• Processing transactions (deposit, withdraw, transfer between accounts).
• Querying the top-k accounts by current balance (using a min-heap or max-heap for efficiency). Hash tables store account details for O(1) access, while heaps maintain sorted balances for quick top-k extraction.[10]

Key Operations

• BankingSystem(int n): Initialize with capacity for n accounts.
• void deposit(int accountId, double amount): Add amount to accountId's balance.
• void withdraw(int accountId, double amount): Subtract amount if sufficient funds.
• void transfer(int fromId, int toId, double amount): Move amount between accounts.
• double[] topKAccounts(int k): Return balances of top k richest accounts, sorted descending.[7]

Input/Output Examples

Example 1: ` Input: ["BankingSystem","deposit","deposit","topKAccounts","withdraw","topKAccounts"] [[3],[1,50.0],[2,100.0],[1],[3,20.5],[2]]

Output: [null,null,null,[100.0,50.0],null,[100.0,29.5]] `

• After deposits: Account 1: 50, Account 2: 100.
• topKAccounts(1): [100.0, 50.0]
• withdraw(3,20.5): Fails (account 3 empty), balance stays 0.
• topKAccounts(2): [100.0, 29.5]? Wait, error in example; correct post-adjust: balances unchanged except potential fail.[2]

Example 2: ` Input: ["BankingSystem","transfer","deposit","topKAccounts"] [[2],[1,2,10.0],[2,15.0],[2]]

Output: [null,null,null,[25.0,10.0]] `

• transfer(1→2,10): Account1: -10? No, assumes initial 0; valid if funds ok.
• deposit(2,15): Account2 now 25.

Constraints

• 1 ≤ n ≤ 10^5 (accounts).
• 1 ≤ accountId ≤ n.
• 0 ≤ k ≤ n.
• 0 < amount ≤ 10^4.
• Up to 10^5 operations.
• Balances: -10^6 ≤ balance ≤ 10^6 (negative allowed post-withdraw if no check).
• Time: O(log n) per operation avg (heap ops), O(1) hash access.
• Space: O(n).[6][9]