9.11

学到了编程的精义，编程思维：分解、模式识别、抽象、算法、单元测试、整合
回顾了random的使用


import java.util.Random;

public class Number {
    public static void main(String[] args) {
        Random random = new Random();

        for (int i = 0; i < 30; i++) {
            int n1 = random.nextInt(90) + 10;
            int n2 = random.nextInt(90) + 10;
            String[] operators = {"+", "-", "*", "/"};
            String operator = operators[random.nextInt(4)];
            String arithmeticExpression = n1 + " " + operator + " " + n2 + " = ";

            System.out.println(arithmeticExpression);
        }
    }
}