GPU Credits II (Modified): Balance Query Returns None When Subtract Fails

Problem: GPU Credits II (Modified) Implement a simple GPU credit account system that supports adding credits, subtracting credits, and querying balances. Required API Design a class/module that supports at least: add(user, amount): add amount credits to user. subtract(user, amount): subtract amount credits from user. If the current balance is insufficient, the subtraction must not take effect. getBalance(user): return the current balance for user. Modification: when the “current credits are not enough to subtract” (i.e., the most recent subtraction attempt for that user failed due to insufficient balance), getBalance(user) must return None. Otherwise return the integer balance. No need to optimize for time complexity; a clear correct implementation is sufficient. I/O (one possible judge format) Read commands from stdin and print the result of each GET on its own line: ADD user amount SUB user amount GET user Assume: user is a string without spaces amount is a non-negative integer Constraints Number of commands N: 1 to 2e5 amount: 0 to 1e9 Sample Tests input: ADD alice 10 GET alice output: 10 input: ADD alice 10 SUB alice 3 GET alice output: 7 input: ADD alice 5 SUB alice 10 GET alice output: None input: ADD bob 0 GET bob SUB bob 1 GET bob output: 0 None input: ADD alice 10 SUB alice 6 SUB alice 5 GET alice ADD alice 1 GET alice output: None 5 Example Input ADD alice 10 GET alice Output 10