题解 | 牛牛学数列6

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static int an(int n){
        int res = 0;

        if(n == 1 )return 0;
        if(n == 2 || n == 3)return 1;
        if(n >= 4){
            res = an(n - 3) + 2 * an(n -2) + an(n -1 );
        }
        
        return res;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        if(n == 1){
            System.out.println(0);
        }else if( n == 2 || n == 3){

            System.out.println(1);
        }else{
            int res = 0; 
            res = an(n);
            System.out.println(res);
        }
    }
}