矩阵快速幂

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N = 3;
int n,m;

void mul(int c[],int a[],int b[][N])
{
    int temp[N] = {0};
    for(int i = 0;i<N;i++)
    {
        for(int j = 0;j<N;j++)
        {
            temp[i] = (temp[i] + a[j]*b[j][i])%m;
        }
    }
    memcpy(c,temp,sizeof temp);
}

void mul(int c[][N],int a[][N],int b[][N])
{
    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] + a[i][k] * b[k][j]) % m;
            }
        }
    }
    memcpy(c,temp,sizeof temp);
}

signed main()
{
    cin>>n>>m;
    int f1[3] = {1,1,1};
    int a[N][N] = {
        {0,1,0},
        {1,1,1},
        {0,0,1},
    };

    n--;
    while(n)
    {
        if(n&1)mul(f1,f1,a);
        mul(a,a,a);
        n >>= 1;
    }
    cout<<f1[2]<<endl;
    return 0;
}