Find the longest common prefix string amongst an array of strings. Return the length of this common prefix.

Implement a basic calculator to evaluate a simple expression string. The expression can contain non-negative integers, '+', '-', '*', '/' operators, and spaces; integer division should truncate toward zero.

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.