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 void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        if(n < 1 || n > 20){
            System.out.print("no use");
        }else if(n == 1){
            System.out.print(0);
        }else if(n == 2 || n ==3){
            System.out.print(0);
        }else if(n >= 4){
            int an = intzhi(n);
            System.out.print(an);
        }
    }
    public static int intzhi(int i){
        int[] arr = new int[i];
        arr[0] = 0;
        arr[1] = 1;
        arr[2] = 1;
        for(int j = 3; j < i; j++){
            arr[j] = arr[j-3] + 2*arr[j-2] + arr[j-1];
        }
        return arr[arr.length - 1];
    }
}