题解 | 牛牛学数列6

#include <iostream>
#include <vector>
using namespace std;

int main() 
{
    int n;
    cin>>n;
    if(n == 1)
    {
        cout<<0;
    }
    else if(n == 2 || n == 3)
    {
        cout<<1;
    }
    else 
    {
        vector<int> v;
        v.push_back(0);
        v.push_back(1);
        v.push_back(1);
        int num = 0;
        for(int i = 3;i<n;++i)
        {
            num = v[i-3]+2*v[i-2]+v[i-1];
            v.push_back(num);
        }    
        cout<<v[n-1];
    }
}
// 64 位输出请用 printf("%lld")