一些 OO Design 面试题

设计 hash map

解决方案
Constraints and assumptions：

• For simplicity, are the keys integers only? Yes
• For collision resolution, can we use chaining? Yes
• Do we have to worry about load factors? No
• Can we assume inputs are valid or do we have to validate them? Assume they're valid
• Can we assume this fits memory? Yes

具体实现比较简单，不赘述。

设计 LRU 缓存

解决方案
Constraints and assumptions：

• What are we caching? We are cahing the results of web queries
• Can we assume inputs are valid or do we have to validate them? Assume they're valid
• Can we assume this fits memory? Yes

class Node {
    int key;
    int value;
    
    Node pre;
    Node next;

    public Node(int key, int value) {
        this.key = key;
        this.value = value;
    }
}

public class LRUCache {
    HashMap<Integer, Node> map;
    int capicity;
    int count;
    
    // 构成一个双向链表
    Node head, tail;
    
    public LRUCache(int capacity) {
        map = new HashMap<>();
        
        this.capicity = capacity;
        count = 0;
        
        head = new Node(0, 0);
        tail = new Node(0, 0);
        
        head.next = tail;
        head.pre = null;
        
        tail.pre = head;
        tail.next = null;
    }
    
    public void deleteNode(Node node) {
        node.pre.next = node.next;
        node.next.pre = node.pre;
    }
    
    public void addToHead(Node node) {
        node.next = head.next;
        node.next.pre = node;
        node.pre = head;
        head.next = node;
    }
    
    public int get(int key) {
        if (map.get(key) != null) {
            Node node = map.get(key);
            int result = node.value;
            
            // 从当前位置移除，放到头部
            deleteNode(node);
            addToHead(node);
            
            return result;
        }
        
        return -1;
    }
    
    public void put(int key, int value) {
        // 更新
        if (map.get(key) != null) {
            Node node = map.get(key);
            node.value = value;
            
            // 从当前位置移除，放到头部
            deleteNode(node);
            addToHead(node);
        }
        // 加入
        else {
            Node node = new Node(key, value);
            map.put(key, node);
            // 放到头部
            addToHead(node);
            
            // 没有达到capicity
            if (count < capicity) {
                count++;
            } else {
                // 移除队尾
                map.remove(tail.pre.key);
                deleteNode(tail.pre);
            }
        }
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */

设计一个呼叫中心

解决方案
Constraints and assumptions：

• What levels of employees are in the call center?
  Operator, supervisor, director
• Can we assume operators always get the initial calls?
  Yes
• If there is no free operators or the operator can't handle the call, does the call go to the supervisors?
  Yes
• If there is no free supervisors or the supervisor can't handle the call, does the call go to the directors?
  Yes
• Can we assume the directors can handle all calls?
  Yes
• What happens if nobody can answer the call?
  It gets queued
• Do we need to handle 'VIP' calls where we put someone to the front of the line?
  No
• Can we assume inputs are valid or do we have to validate them?
  Assume they're valid

设计一副牌

解决方案
Constraints and assumptions：

• Is this a generic deck of cards for games like poker and black jack?
  Yes, design a generic deck then extend it to black jack
• Can we assume the deck has 52 cards (2-10, Jack, Queen, King, Ace) and 4 suits?
  Yes
• Can we assume inputs are valid or do we have to validate them?
  Assume they're valid

设计一个停车场

解决方案
Constraints and assumptions：

• What types of vehicles should we support?
  Motorcycle, Car, Bus
• Does each vehicle type take up a different amount of parking spots?
  Yes
  Motorcycle spot -> Motorcycle
  Compact spot -> Motorcycle, Car
  Large spot -> Motorcycle, Car
  Bus can park if we have 5 consecutive "large" spots
• Does the parking lot have multiple levels?
  Yes

基本思想：

• 定义一个枚举类型 VehicleSize 来标记不同的交通工具种类。
• 定义一个抽象类 Vehicle 来表示交通工具，包括种类，车牌号等熟悉。
• 依次定义 Vehicle 的实现类，并实现各自的 can_fit_in_spot 方法：
  • Motorcycle
  • Car
  • Bus
• 定义停车场 ParkingLot，它包含很多层 self.levels = []。停车时，以此遍历每一层，停车。
• 定义每一层 Level，它包含很多停车位 self.parking_spots = []。
• 定义每一个停车位 ParkingSpot，包含编号，尺寸，是否停有车辆等属性。

设计一个聊天服务

解决方案
Constraints and assumptions：

• Text conversations only 只支持文本消息
• Users 用户的管理
  • Add a user
  • Remove a user
  • Update a user
  • Add to a user's friends list
    • Add friend request
    • Approve friend request
    • Reject friend request
  • Remove from a user's friends list
• Create a group chat 群聊
  • Invite friends to a group chat
  • Post a message to a group chat
• Private 1-1 chat 单独聊
  • Invite a friend to a private chat
  • Post a meesage to a private chat
• No need to worry about scaling initially

基本思想：

• 定义一个用户类 User，包含：

  • 基本信息，例如用户名，密码
  • 好友信息 # key: friend id, value: User
  • 发送出去的添加好友请求 # key: friend id, value: AddRequest
  • 接受进来的添加好友请求 # key: friend id, value: AddRequest
  • 正在进行的单聊 # key: friend id, value: private chats
  • 正在进行的群聊 # key: chat id, value: GroupChat

• 定义 UserService 来处理用户相关的操作，包括：

  • 添加及删除用户
  • 发送添加好友请求，批准请求，拒绝请求

• 定义 AddRequest 来表示添加好友的请求，包括发送人，接收人，发送时间，状态等属性。

• 定义 Message 来表示聊天消息，包括内容，发送时间，发送人等属性。

• 定义抽象类 Chat 来表示聊天，包括：

  • 编号
  • 有哪些人 self.users = []
  • 聊天消息 self.messages = []

• 定义 Chat 的两个具体实现：

  • PrivateChat
  • GroupChat