出四则运算题，可任意操作数

应付作业写的漏洞太多谨慎观看

import java.util.Random;
import java.lang.Math;
import java.util.Scanner;

public class RandomCommon {
    public static int[] randomCommon(int min, int max, int sumoprand){
        int sum = sumoprand;
        if (sum > (max - min + 1) || max < min) {
            return null;
        }
        int[] result = new int[sum];
        int count = 0;
        while(count < sum) {
            int num = (int) (Math.random() * (max - min)) + min;
            boolean flag = true;
            for (int j = 0; j < sum; j++) {
                if(num == result[j]){
                    flag = false;
                    break;
                }
            }
            if(flag){
                result[count] = num;
                count++;
            }
        }
        return result;
    }
    
    public static char operator(int n,int se) {
        char op=' ';
        Random random = new Random();
        n = random.nextInt(100);
        if(se==1) 
        {
            switch(n%4)
            {
            case 0:op = '+';break;
            case 1:op = '-';break;
            case 2:op = '*';break;
            case 3:op = '/';break;
            }
        }
        if(se==2)
        {
            switch(n%2)
            {
            case 0:op = '+';break;
            case 1:op = '-';break;
            }
        }
        return op;
    }
    
    public static void main(String[] args) {
        int sumtitle;//出题个数
        int sumoperand;//操作数个数
        int min,max;
        int chengchu;
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入出题个数:");
        sumtitle = sc.nextInt();
        System.out.println("请输入操作数个数：");
        sumoperand = sc.nextInt();
        System.out.println("请输入任意数范围：");
        System.out.println("min=");
        min = sc.nextInt();
        System.out.println("max=");
        max = sc.nextInt();
        System.out.println("是否需要乘除法：1.需要 2.不需要");
        chengchu = sc.nextInt();
        String[] str = new String[sumtitle];
        String str0;
        Random random = new Random();
        for(int i=0;i<sumtitle;i++)
        {
            int a = random.nextInt(max);
            int b = random.nextInt(max);
            int cc = random.nextInt(100);
            char c = operator(cc,chengchu);
            str0 = ""+a+c+b;
            str[i] = problem(str0,sumoperand,min,max,sumoperand,chengchu);
        }
        for(int i=0;i<sumtitle;i++)
        {
            System.out.println(str[i]+"\n");
        }
    }
    
    public static String problem(String str,int n,int min,int max,int sumoprand,int chengchu) {
        Random random = new Random();
        int b = random.nextInt(100);
        int[] arr = randomCommon(min,max,sumoprand);
        for(int i=2;i<n;i++)
        {
            char c = operator(b,chengchu);
            str = str + c + arr[i];
        }
        return str;
    }
    
}