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详细说明：Implement a phone book system for employees of a company.
Your program will output the following menu
(1) Enter an employee and a phone pair to the system
(2) Lookup an employee s phone number
(3) Find out who is/are the person(s) of a given number
(4) How many people are currently in the system
(5) Delete an employee from the system
(6) Output all employees name‐phone pair
(7) How many phone numbers total in the current system
(8) Quit
When