题解 | 牛牛学数列6

#include <iostream>
using namespace std;


int main() {
    int n;
    cin>>n;
    if(n==1){cout<<0<<endl;return 0;}
    if(n==2 || n==3 ){cout<<1<<endl;return 0;}

    int a1 = 0,a2 =1,a3=1,an;
    for(int i=4;i<=n;i++){
        an = a1+2*a2 + a3;
        a1 = a2;
        a2 = a3;
        a3 = an;
    }
    cout<<an<<endl;
}

#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;

    int a[25];
    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;
}