题解 | #牛牛学数列6#

简单递推

#include<stdio.h>
int n,a[21]={0,0,1,1};
int main(){
    scanf("%d",&n);
    for(int i=4;i<=n;i++) a[i]=a[i-3]+2*a[i-2]+a[i-1];
    printf("%d",a[n]);
}