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);
        int n = in.nextInt();
        if(n == 1){
            System.out.println(0);
        }
        else if(n == 2||n==3){
            System.out.println(1);
        }
        else{
            int[] s = new int[n];
            s[0] = 0;
            s[1] = 1;
            s[2] = 1;
            for(int i = 3;i<n;i++){
                s[i] = s[i-3]+2*s[i-2]+s[i-1];
            }
            System.out.println(s[n-1]);
        }
    }
}