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();
        System.out.printf("%d",an(n));
        
       
       
}
public static int an( int n){
     
            if(n==1){
               return 0;
            }
            if(n==2||n==3){
                return 1;
            }else{
                int a = 0;
                int b = 1;
                int c = 1;
                int d = 0;
            for(int i =4;i<=n;i++){
                d=a+2*b+c;
                a=b;
                b=c;
                c=d;

                

}
           return d;
}

}
}