题解 | 牛牛学数列6

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

int solve(int x)
{
    if(x == 1) return 0;
    if(x == 2 || x == 3) return 1;
    return solve(x - 3) + 2 * solve(x - 2 ) + solve(x - 1);
}
int main() {   

    int n;
    cin >> n;
    cout << solve(n);
  
    return 0;
}
// 64 位输出请用 printf("%lld")