組合數取模

#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9+7;
const int maxn = 1e5+10;
typedef long long LL;
LL fac[maxn];
LL modxp(LL a,LL b)
{
    LL res = 1;
    while(b)
    {
        if(b&1)
            res = (res*a)%mod;
        b = b>>1;
        a = (a*a)%mod;
    }
    return res;
}
void Init()
{
    fac[0] = 1;
    for(int i = 1; i < maxn; i++)
    {
        fac[i] = (fac[i-1]*i)%mod;
    }
}
LL C(LL a,LL k)
{
    LL re = 1;
    while(a && k)
    {
        LL aa = a%mod;
        LL bb = k%mod;
        if(aa<bb)
            return 0;
        re = re*fac[aa]*modxp(fac[bb]*fac[aa-bb]%mod,mod-2)%mod;
        a /= mod;
        k /= mod;
    }
    return re;
}
int main()
{
    Init();
    LL a, b;
    cin >> a >> b;
    cout << C(a, b) << endl;
    return 0;
}