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();
        // int[] arr = new int[n+1];
        // arr[1] = 0;
        // arr[2] = 1;
        // arr[3] = 1;
        // if(n>=4){
        //     for(int i = 4;i<=n;i++){
        //         arr[i] = arr[i-3]+2*arr[i-2]+arr[i-1];
        //     }
        // }
        // System.out.println(arr[n]);

        int a1 = 0;
        int a2 = 1;
        int a3 = 1;
        int an = 0;
        if(n==1) System.out.println(a1);
        if(n==2 || n==3) System.out.println(a2);
        for(int i = 4;i<=n;i++){
            an = a1+2*a2+a3;
            a1 = a2;
            a2 = a3;
            a3 = an;
        }
        System.out.println(an);
    }
}