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
        while (in.hasNextInt()) { // 注意 while 处理多个 case
            int a = in.nextInt();
            int n1 = 0, n2 = 1, n3 = 1;
            int n;
            if(a == 1) {
                n = 0;
            } else if(a == 2 || a == 3) {
                n = 1;
            } else if(a >= 4) {
                for(int i = 4; i <= a; i++) {
                    int temp = n3;
                    n3 = n1 + 2 * n2 + n3;
                    n1 = n2;
                    n2 = temp;
                }
                System.out.println(n3);
            } else {
                System.out.println("请输入1到20内的任意数字！");
            }         
        }
    }
}