題一

Imagine you have a call center with three levels of employees: fresher, technical lead (TL), product manager (PM). There can be multiple employees, but only one TL or PM.
An incoming telephone call must be allocated to a fresher who is free. If a fresher can’t handle the call, he or she must escalate the call to technical lead. If the TL is not free or not able to handle it, then the call should be escalated to PM. Design the classes and data structures for this problem. Implement a method getCallHandler().

分析一下，我們大概可以確定有如下類：

call 這個類應該有

rank： 這個call的級別

content： 這個call的內容

reply(): 回覆這個call

disconnect()：結束通話這個call

employee 這個類有僱員的一般資訊(這裡就不考慮這個了). 同時應該有幾個重要屬性和方法：

rank:僱員可以處理的事物級別。

free：僱員此刻是否空閒。

receiveCall(): 僱員被派發了一個call。

canHandleCall(): 僱員處理一個call。

canNotHandleCall(): 僱員無法處理一個call時。這裡會選擇將call的級別加一，並再次dispatch這個call。

fresher，technicalLead，productMmanager。這3個類應該繼承employee類。其中兩個類可以考慮使用singleton模式保證只能例項化一個例項。

callHandler 這個類應該實現題目中要求的getCallHandler()方法，這個方法返回一個符合要求的僱員。

例項化不同的僱員。

取得一個call

派發一個call，這個方法從getCallHandler()得到一個合適的僱員，並呼叫他的receiveCall()。

public class CallHandler{
    static final int LEVELS = 3;
    static final int NUM_FRESHERS = 5;
    ArrayList<Employee>[] employeeLevels = new ArrayList[LEVELS];
    // queues for each call’s rank
    Queue<Call>[] callQueues = new LinkedList[LEVELS];

    public CallHandler() { ... }

    Employee getCallHandler(Call call) {
        for (int level = call.rank; level < LEVELS - 1; level++) {
            ArrayList<Employee> employeeLevel = employeeLevels[level];
            for (Employee emp : employeeLevel) {
                if (emp.free) {
                return emp;
                }
            }
        }
        return null;
    }

    // routes the call to an available employee, or adds to a queue
    void dispatchCall(Call call) {
        // try to route the call to an employee with minimal rank
        Employee emp = getCallHandler(call);
        if (emp != null) {
            emp.ReceiveCall(call);
        } else {
        // place the call into queue according to its rank
            callQueues[call.rank].add(call);
        }
    }
    void getNextCall(Employee e) {...} // look for call for e’s rank
    }

    class Call {
        int rank = 0; // minimal rank of employee who can handle this call
        public void reply(String message) { ... }
        public void disconnect() { ... }
    }

    class Employee {
        CallHandler callHandler;
        int rank; // 0- fresher, - technical lead, - product manager
        boolean free;
        Employee(int rank) { this.rank = rank; }
        void ReceiveCall(Call call) { ... }
        void CallHandled(Call call) { ... } // call is complete
        void CannotHandle(Call call) { // escalate call
            call.rank = rank + 1;
            callHandler.dispatchCall(call);
            free = true;
            callHandler.getNextCall(this); // look for waiting call
        }
    }

    class Fresher extends Employee {
        public Fresher() { super(0); }
    }
    class TechLead extends Employee {
        public TechLead() { super(1); }
    }
    class ProductManager extends Employee {
        public ProductManager() { super(2); }
    }

題二

題三
Explain how you would design a chat server. In particular, provide details about the various backend components, classes, and methods. What would be the hardest
problems to solve?

這設計題並不難，需要面試的時候和麵試人溝通，確定更多的細節。這裡我們可以假設

題四

Explain the data structures and algorithms that you would use to design an in-memory !le system. Illustrate with an example in code where possible.

大家先看看，之後再補充答案。