51Nod 1126 求递推序列的第N项（矩阵快速幂）

#include <iostream>
#include <algorithm>
#include <cmath>
#define MOD 7 
#define N 2
using namespace std;

struct Matrix
{
    long long v[N][N];
};

//矩阵间的乘法%m
Matrix matrix_mul(Matrix A, Matrix B, long long m)
{
    Matrix ans;

    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            ans.v[i][j] = 0;
            for (int k = 0; k < N; k++)
            {
                ans.v[i][j] += (A.v[i][k] * B.v[k][j]) % m;
            }
            ans.v[i][j] %= m;
        }
    }
    return ans;
}

//矩阵快速幂c^n%m
Matrix matrix_pow(Matrix C, long long n, long long m)
{
    Matrix ans = { 1, 0, 0, 1 };//e
    while (n)
    {
        if (n & 1)
            ans = matrix_mul(ans, C, m);
        C = matrix_mul(C, C, m);
        n >>= 1;
    }
    return ans;
}

int main()
{
    long long n;
    int a, b;
    cin >> a >> b >> n;
    Matrix temp1 = { (a%MOD + MOD) % MOD, (b%MOD + MOD) % MOD, 1, 0 };
    Matrix temp2 = { 1, 0, 1, 0 }; // temp2{f[2],0,f[1],0}!!!!!

    if (n < 2)
    {
        cout << 1 << endl;
        return 0;
    }
    Matrix res = matrix_pow(temp1, n - 2, MOD);
    res = matrix_mul(res, temp2, MOD);
    cout << res.v[0][0] << endl;
    return 0;
}