這裡寫的很詳細了啊：http://blog.csdn.net/acmmmm/article/details/9722515?utm_source=tuicool

記錄一下，回頭複習用啊。

So Easy!

　　A sequence S_n is defined as:

Where a, b, n, m are positive integers.┌x┐is the ceil of x. For example, ┌3.14┐=4. You are to calculate S_n.
　　You, a top coder, say: So easy! 

#include <algorithm>
#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <iomanip>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>
#include <stack>
#include <map>
#include <set>
#define eps 1e-10
///#define M 1000100
#define LL __int64
///#define LL long long
///#define INF 0x7ffffff
#define INF 0x3f3f3f3f
#define PI 3.1415926535898
#define zero(x) ((fabs(x)<eps)?0:x)

///#define mod 10007

const int maxn = 210;

using namespace std;

struct matrix
{
    LL f[3][3];
};
LL mod;

matrix mul(matrix a, matrix b, int n)
{
    matrix c;
    memset(c.f, 0, sizeof(c.f));
    for(int k = 0;k < n; k++)
    {
        for(int i = 0; i < n;i++)
        {
            if(!a.f[i][k]) continue;
            for(int j = 0; j < n; j++)
            {
                if(!b.f[k][j]) continue;
                c.f[i][j]=(c.f[i][j]+a.f[i][k]*b.f[k][j]+mod)%mod;
            }
        }
    }
    return c;
}
matrix pow_mod(matrix a, LL b, int n)
{
    matrix s;
    memset(s.f, 0 , sizeof(s.f));
    for(int i = 0; i < n; i++) s.f[i][i] = 1LL;
    while(b)
    {
        if(b&1) s = mul(s, a, n);
        a = mul(a, a, n);
        b >>= 1;
    }
    return s;
}

int main()
{
    LL a, b, n;
    while(~scanf("%I64d %I64d %I64d %I64d",&a, &b, &n, &mod))
    {
        if(n == 1)
        {
            printf("%I64d\n",2*a%mod);
            continue;
        }
        matrix c;
        memset(c.f, 0 , sizeof(c.f));
        c.f[0][0] = 2LL*a;
        c.f[0][1] = b-a*a;
        c.f[1][0] = 1LL;
        matrix d = pow_mod(c, n-1, 2);
        printf("%I64d\n",((d.f[0][0]*2*a+d.f[0][1]*2)%mod+mod)%mod);
    }
    return 0;
}