斐波那契前 n 项和

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

typedef long long LL;

const int N = 3;

int n, m;

void mul(int c[], int a[], int b[][N])//一维等于一维乘以二维
{
    int temp[N] = {0};//定义一个临时指针
    for (int i = 0; i < N; i ++ )
        for (int j = 0; j < N; j ++ )
            temp[i] = (temp[i] + (LL)a[j] * b[j][i]) % m;

    memcpy(c, temp, sizeof temp);
}

void mul(int c[][N], int a[][N], int b[][N])//矩阵乘法，二维 = 二维乘以二维
{
    int temp[N][N] = {0};
    for (int i = 0; i < N; i ++ )
        for (int j = 0; j < N; j ++ )
            for (int k = 0; k < N; k ++ )
                temp[i][j] = (temp[i][j] + (LL)a[i][k] * b[k][j]) % m;

    memcpy(c, temp, sizeof temp);
}

int main()
{
    cin >> n >> m;

    int f1[N] = {1, 1, 1};
    int a[N][N] = {、、出事矩阵
        {0, 1, 0},
        {1, 1, 1},
        {0, 0, 1}
    };

    n -- ;
    while (n)//快速幂，矩阵A^(n-1)
    {
        if (n & 1) mul(f1, f1, a);  // res = res * a
        mul(a, a, a);  // a = a * a
        n >>= 1;
    }

    cout << f1[2] << endl;

    return 0;
}