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 A_n = 0;
        if(n >= 1 && n <= 20){
            if(n == 1){
                 A_n = 0;
            }else if(n == 2 || n == 3){
                 A_n = 1;
                 A_n = 1;
            }else{
                int a = 0;
                int b = 1;
                int c = 1;
                for(int i = 4;i <= n;i++ ){
                    A_n = a + 2 * b + c;
                    a = b;
                    b = c;
                    c = A_n;
                }
            }
            System.out.println(A_n);
        }
    }
}