练习cf450B. Jzzhu and Sequences

题目如下
B. Jzzhu and Sequences
time limit per test1 second
memory limit per test256 megabytes
Jzzhu has invented a kind of sequences, they meet the following property:

You are given x and y, please calculate fn modulo 1000000007 (109 + 7).

Input
The first line contains two integers x and y (|x|, |y| ≤ 109). The second line contains a single integer n (1 ≤ n ≤ 2·109).

Output
Output a single integer representing fn modulo 1000000007 (109 + 7).

题目大意
根据fi = fi-1 + fi+1 ,对fn进行取模并输出。

题目分析
根据fi = fi-1 + fi+1 推出 fi = fi-1 - fi-2（i>=3)，从后往前推发现存在周期为6的循环，那么

点击查看代码

#include <iostream>
using namespace std;

int main(){
    long long x, y, n;
    cin >> x >> y;
    cin >> n;
    long long ans, res;
    long long  k= (n - 1) % 6;
    if(k == 0){
        ans = x;
    }else if(k == 1){
        ans = y;
    }else if(k == 2){
        ans = y - x;
    }else if (k == 3){
        ans = -x;
    }else if (k == 4){
        ans = -y;
    }else{
        ans = x - y;
    }
    res = ans % 1000000007;
    if(res < 0){
        res += 1000000007;
    }
    cout << res << "";
    return 0;
}