编写一个账户管理类AccountManager，使用映射容器管理账户，其中以字符串表示的账户名是键对象，而以前编写的类Account是值对象。类AccountManager应该具备开设账户、删除账户、通过户名查找账户余额、列出所有账户、清除余额小于1元的账户、统计所有账户的总余额、查找最大余额和最小余额的账户等功能。

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Write an account management class AccountManager, using the mapping container-managed accounts, which account name is the string representation of the key object, and the previous value written is an object of the class Account. AccountManager class should have to open an account, delete the account, username lookup through account balances, list all accounts, clearing account balance is less than $ 1, the total balance of all accounts statistics, find the maximum and minimum balance account balance and other functions