Simulate a simple bank system that supports basic operations like deposit, withdraw, and check balance for different accounts. You will implement methods to handle these transactions and ensure validity based on account balances and existence.

Implement an iterator that cycles through multiple lists in a zigzag pattern. The iterator should return elements from each list in turn until all elements are exhausted.

Design a key-value store that supports storing values with timestamps. You should be able to retrieve the latest value for a key that's less than or equal to a given timestamp.