组合数取模(Lucas定理)

模数为p=9901

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<iostream>
#define ll long long
using namespace std;
int m,n,p=9901;
ll modfac[10000]={1,1};//1-n的阶乘取模后结果
ll fp(ll a,ll b){
    if(b==1)
    return a;
    ll i=fp(a,b/2);
    if(b%2==0)
    return i*i%p;
    else
    return i*i*a%p;
}
ll c(ll n,ll m)
{
    if(m>n)
        return 0;
    return (modfac[n]*fp(modfac[n-m],p-2)*fp(modfac[m],p-2))%p;
}
ll lucas(ll a,ll b){
    if(b==0) 
    return 1;
    else
    return c(a%p,b%p)*lucas(a/p,b/p)%p;
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=2;i<=min(9905,n);i++){
        modfac[i]=modfac[i-1]*i%p;
    }
    printf("%lld",lucas(n,m));
    return 0;
}