题解 | 牛牛学数列6

#include<bits/stdc++.h>
using namespace std;

const int N=30;

int a[N];

int main(){
	int n;
	cin>>n;
	
	if(n==1){
		cout<<0<<endl;
	}else if(n==2||n==3){
		cout<<1<<endl;
	}else{
		a[1]=0;a[2]=1;a[3]=1;
		for(int i=4;i<=n;i++){
			a[i]=a[i-3]+2*a[i-2]+a[i-1];
		}
		cout<<a[n]<<endl;
	}

    return 0;
}