BZOJ 4870 [Shoi2017]组合数问题 ——动态规划 矩阵乘法

#include <map>
#include <cmath>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define F(i,j,k) for (int i=j;i<=k;++i)
#define D(i,j,k) for (int i=j;i>=k;--i)
#define ll long long
#define mp make_pair
  
int n,p,k,r;
ll c;
  
struct Matrix{
    int x[51][51];
    void init(){memset(x,0,sizeof x);return;}
    void buildt()
    {init();F(i,0,k-1)x[i][i]++,x[i][(i+1)%k]++;return;}
    void builds()
    {init();x[0][0]=1;return;}
    Matrix operator * (Matrix a) {
        Matrix ret;
        ret.init();
        F(i,0,k-1) F(j,0,k-1) F(l,0,k-1)
            ret.x[i][j]=(ret.x[i][j]+(ll)x[i][l]*a.x[l][j])%p;
        return ret;
    }
}A,B;
  
int main()
{
    scanf("%d%d%d%d",&n,&p,&k,&r); c=(ll)n*k;
    A.builds(); B.buildt();
    for (;c;c>>=1,B=B*B) if (c&1LL) A=A*B;
    printf("%d\n",A.x[0][r]);
}