逆序数是大学高等代数的一个比较简单的概念，整体思路很简单，通过数组去接受一个n维的数据，遍历去获取后方比自己小的数，累加即可。

下方为完整代码

import java.util.Scanner;

public class Reverse_order_number {
    public static void main(String[] args) {
        int[] a = Way.input();
        //Way.print(a);
        System.out.println(" ");
        int answer = Way.get1(a);
        System.out.println("逆序数为" + answer);
    }
}
class Way{
    public static int[] input(){
        Scanner scanner = new Scanner(System.in);
        System.out.println("输入阶数");
        int n = scanner.nextInt();
        int[] var = new int[n];
        for (int i = 0; i < var.length; i++) {
            System.out.println("输入第"+(i+1)+"位数");
            var[i] = scanner.nextInt();
        }
        return var;
    }

    public static int get1(int[] a){
        int num = 0;
        for (int i = 0; i < a.length; i++) {
            //System.out.print(check1(a,a[i])+"+");
            num+=check1(a,a[i]);
        }
        return num;
    }

    public static int check1(int[] a, int b){
        int num = 0;
        for (int i = check(a,b); i < a.length; i++) {
            if(a[i]<b){
                num++;
            }
        }
        System.out.println(b+"后面之后有"+num+"个比"+b+"小的");
        return num;
    }

    public static int check(int[] a ,int b){
        for (int i = 0; i < a.length; i++) {
            if (a[i] == b){
                return i;
            }
        }
        return -1;
    }
}