题解 | 牛牛学数列6

一、不带数组解法
#include <iostream>
using namespace std;

int main() {
    int a;
    cin >> a;
    int b = 1 , c = 1 , d = 0 , e;
    if(a == 1) cout << d << endl;
    if(a == 2 || a == 3) cout << b << endl;
    if(a >= 4)
    {
        for(int i = 4;i <= a;i++)
        {
            e = b;
            b = b + 2 * c + d;
            d = c;
            c = e;
        }
    }
    cout << b << endl;
    return 0;
}
二、数组解法
#include <iostream>
using namespace std;

int main() {
    int a;
    cin >> a;
    int b[100];
    b[0] = 0;
    b[1] = b[2] = 1;
    for (int i = 3; i < a; i++)
    {
        b[i] = b[i - 1] + 2 * b[i - 2] + b[i - 3];
    }
    cout << b[a - 1] << endl;
    return 0;
}