hud 2502 月之数

I think:

AC :

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        while(n--!=0)
        {
            int a=sc.nextInt();
            System.out.println(""+f(a));
        }
    }
    public static int f(int a)
    {
        int [] a1=new int [22];
        a1[0]=1;int b=1;
        for(int i=1;i<=20;i++)
        {    a1[i]=a1[i-1]*2+b;    b <<= 1;    }
        return a1[a-1];
    }
}