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 sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        int sum = 0;
        arr[0] = 0;
        arr[1] = 1;
        arr[2] = 1;
        for (int i = 3; i < n; i++) {
            arr[i] = arr[i - 1] + 2 *  arr[i - 2] + arr[i - 3];
            sum = arr[i];
        }
        System.out.println(sum);
    }

}