题解 | 牛牛学数列6

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);     
            int a = in.nextInt();
            int []arr=new int[a+1];
            arr[1]=0;
            arr[2]=arr[3]=1;
            for(int i=4;i<=a;i++){
                arr[i]=arr[i-3]+2*arr[i-2]+arr[i-1];
            }
        System.out.println(arr[a]);
        }
    }