Algorithm
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- Mock Interview from Leetcode">Mock Interview from Leetcode
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- Java Basics
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  - Instance Fields
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- How to get a Testing Job in New Zealand
- Angular、React 和 Vue 三大框架，Web 开发该如何选择？">Angular、React 和 Vue 三大框架，Web 开发该如何选择？

Algorithm

Factorial Numbers阶乘

def FAC(n):
    if n == 0:
        return 1
    return FAC(n - 1) * n
def FAC2(n):
    result = 1
    while n > 0:
        result = result * n
        n = n - 1
    return result
def test():
    print(FAC(5))
    print(FAC2(5))

Fibonacci Numbers斐波那契数列

Dynamic programming (also known as dynamic optimization) is a method for solving a complex problem by breaking it down into a collection of simpler subproblems, solving each of those subproblems just once, and storing their solutions – ideally, using a memory-based data structure.

1. Brute Force，O（n exponential）暴力解法，时间复杂度n的指数级

2. Dynamic Programming, storing the leastest two numbers, O（logn）

引入动态规划思想，时间复杂度降低到logn

//the following 3 functions have the same result
def Fib(n):
    if n == 0:
        return 1
    if n == 1:
        return 1
    return Fib(n - 1) + Fib(n - 2)
def Fib2(n, a=1, b=0):
    if n == 0:
        return a
    return Fib2(n - 1, a + b, a)
def Fib3(n):
    a = 1
    b = 0
    while n > 0:
        a = a + b
        b = a - b
        n = n - 1
    return a
def test():
    print(Fib(15))
    print(Fib2(15))
    print(Fib3(15))

The Tower of Hanoi汉诺塔

递归+数学归纳法思想

Week 12 - 图3
“对递归的理解的要点主要在于放弃!
放弃你对于理解和跟踪递归全程的企图，只理解递归两层之间的交接，以及递归终结的条件。”

Week 12 - 图4
Week 12 - 图5

def Hanoi(n, init, aux, fin):
    if n > 0:
        Hanoi(n - 1, init, fin, aux)//把n-1个从init挪到aux上，借助fin
        fin.append(init.pop())  //把第n个盘子直接从init挪到fin
        print(init)
        print(aux)
        print(fin)
        print("---------")
        Hanoi(n - 1, aux, init, fin)//把n-1个从aux挪到fin上，借助init
def test():
    init = [5, 4, 3, 2, 1]
    fin = []
    aux = []
    Hanoi(5, init, aux, fin)

The Coin Change Problem，分治法

import random

def Ways(amount, denominations):
    if amount == 0:
        return 1
    if amount < 0:
        return 0
    if len(denominations) == 0:
        return 0
    d = random.sample(denominations, 1)[0]   //从denominations产生一个随机数，取第一个
    print(denominations);
    return Ways(amount - d, denominations) + Ways(amount, denominations.difference([d]))
    /*使用d的所有情况+不使用d的所有情况
      denominations.difference([d])表示denominations中删除包含d的元素*/

def test():
    print(Ways(400, set([5, 10, 20, 50, 100, 200])))

Review

Mock Interview from Leetcode

Tip

Java Basics

Java programs have at least one class and one main() method.

Each class represents one real-world idea.
The main() method runs the tasks of the program.
Case-Sensitivity

public is an access level访问级别 modifier修改器 that allows __other classes to interact with this class.

public class Car {
// scope of Car class starts after curly brace   实参

  public static void main(String[] args) {    //声明一个字符串数组,叫args
    // program tasks   形参
  }
}

Output

System is a built-in Java class
out =output
println =print line, the cursor光标 is moved to the next line

println()=outputs everything on the same line

System.out.println("Hello World");
System.out.print("Hello ");

Comment

// calculate customer satisfaction rating
/*We chose to store information across multiple databases to minimize the possibility of data loss. We'll need to be careful to make sure it does not go out of sync!*/

Variables

String name = "James Gosling";
int yearCreated = 1995;
double price = 8.99;
boolean javaIsACupOfCoffee = false;
char punctuation = '!';
String greeting = "Hello World";
String salutations = new String("Hello World");

Escape Sequences转义序列

add quotation
place backslashes反斜杠 \=\
output a new line \n ```java System.out.println(“\”Hello World\””); // Prints: “Hello World”

System.out.println(“This is the backslash symbol: \“); // Prints: This is the backslash symbol: \

System.out.println(“Hello\nGoodbye”); / Prints: Hello Goodbye /

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### Manipluation
equal 

- line1==line2
- line2.equals(line3)

declare a variable with a value that cannot be manipulated

- final int...
```java
int unevenlyDivided = 10 / 4;
//unevenlyDivided holds 2

int songsA = 9;
    int songsB = 9;
    int albumLengthA = 41;
    int albumLengthB = 53;
    boolean sameNumberOfSongs=songsA==songsB;
    boolean differentLength=albumLengthA!=albumLengthB;

final int yearBorn = 1968;

Class Definition

public class Store {
  // instance fields
  String productType;
  int inventoryCount;
  double inventoryPrice;

  // constructor method
  public Store(String product, int count, double price) {
    productType = product;
    inventoryCount = count;
    inventoryPrice = price;
  }

  // main method
  public static void main(String[] args) {
    Store lemonadeStand = new Store("lemonade", 42, .99);
    Store cookieShop = new Store("cookies", 12, 3.75);

    System.out.println("Our first shop sells " + lemonadeStand.productType + " at " + lemonadeStand.inventoryPrice + " per unit.");  
    System.out.println("Our second shop has " + cookieShop.inventoryCount + " units remaining.");
  }
}

Create An Instance

the value of ferrari is a reference to an instance’s memory address
the class Car is used as the ferrari’s type
invoke the constructor method构造函数: Car(), and use new to indicate creating an instance. ```java public class Car { public Car() { // instructions for creating a Car instance } public static void main(String[] args) { // Invoke the constructor Car ferrari = new Car(); } }

public class Car { / declare fields inside the class by specifying the type and name / String color;

public Car() { / instance fields available in scope of constructor method / }

public static void main(String[] args) { // body of main method }

}

public class Car { String color; // constructor method with a parameter

public Car(String carColor) { // parameter value assigned to the field color = carColor; } public static void main(String[] args) { // program tasks } }


- We can initialize a reference-type variable without assigning it a reference if we utilize `null`. If we assign `null` to an object, it would have a void reference.

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###  Constructors

- a class can have multiple constructors as long as they have different parameter values.
```java
public class Car {
  String color;
  int mpg;
  boolean isElectric;

  // constructor 1
  public Car(String carColor, int milesPerGallon) {
    color = carColor;
    mpg = milesPerGallon;
  }
  // constructor 2
  public Car(boolean electricCar, int milesPerGallon) {
    isElectric = electricCar;
    mpg = milesPerGallon;
  }
}

Instance Fields

public class Store {
  // instance fields
  String productType;

  // constructor method
  public Store(String product) {
    productType=product;
  }

  // main method
  public static void main(String[] args) {
    Store lemonadeStand=new Store("lemonade");
    System.out.println(lemonadeStand.productType);
  }
}

How to get a Testing Job in New Zealand

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Angular、React 和 Vue 三大框架，Web 开发该如何选择？

jQuery，使得在 HTML 中编写客户端脚本变得更加容易。
Angular，不需要任何特殊知识，即使你以前从未做过客户端开发，也可以基于以前的 MVC 设计模式使用经验进行构建，这种模式与 MVVM 非常相似。
React，集成 React 时，不需要更改当前项目的代码，它只负责渲染界面，不会额外带来痛苦。
Vue，包含一整套程序，包括 TypeScript 编译器、AOT 编译器和 Web 服务器。Angular 的 Web 服务器用于调试使用这个框架开发的站点。它是用同一个 Angular CLI 实用程序启动的，要启动 Angular CLI，你需要在 Windows 命令行中进入项目文件夹，并执行 ng serve 命令。
Angular 最复杂，有 143KB。React 次之，有 43KB，而 Vue.js 只有 23KB。除非你的应用特别大，并且包含了大量的组件，否则最好使用更小的结构。
React 和 Vue 都实现了 DOM。在 Web 项目中，性能与 DOM 密切相关：DOM 在浏览器 / 代码中表示 Web 页面。在发生更新时，你可以通过 DOM 控制 Web 页面。
社区：React 的 Mental Model 看起来很可靠，其组件让创建用户界面变得更容易，API 灵活且富有表现力
劳动力市场需求：大多数职位空缺与 React.js 有关，然后是 Angular，再然后才是 Vue.js。

专家建议初学者首先学习 Angular，因为你所需的一切都是“开箱即用”的，这样更不容易犯错（框架会帮你控制）。在学习了 Angular 之后，你可以学习 ReactJS 和 React Native。另外，如果你只需要移动应用，你也可以直接跳到 React Native。事实上，你只需要学习框架的特性，仅此而已。其余的知识都是通用的（OOP、TypeScript、RP）。学习框架本身 1 到 2 周就足够了，你已经可以创建简单的 Web 应用程序了。

Week 12