题解 | 牛牛学数列6

#include <iostream>
using namespace std;

int main() {
    int a, b,c;
    int num;
    while (cin >> num) { // 使用迭代法
        if (num < 1 || num > 20) break;
        //已经前三项，abc分别表示前三项,curr表示当前项
        a = 0;
        b = 1;
        c = 1;

        int curr;

        for (int i = 4; i <= num; ++i) {
            curr = a + 2 * b + c;

            //更新迭代
            a = b;
            b = c;
            c = curr;
        }

        cout << curr << endl;

    }
}