hdoj 3037 Saving Beans 【Lucas】

Saving Beans Time Limit: 6000/3000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others) Total Submission(s): 3409    Accepted Submission(s): 1305 Problem Description Although winter is far away, squirrels have to work day and night to save beans. They need plenty of food to get through those long cold days. After some time the squirrel family thinks that they have to solve a problem. They suppose that they will save beans in n different trees. However, since the food is not sufficient nowadays, they will get no more than m beans. They want to know that how many ways there are to save no more than m beans (they are the same) in n trees. Now they turn to you for help, you should give them the answer. The result may be extremely huge; you should output the result modulo p, because squirrels can’t recognize large numbers.   Input The first line contains one integer T, means the number of cases. Then followed T lines, each line contains three integers n, m, p, means that squirrels will save no more than m same beans in n different trees, 1 <= n, m <= 1000000000, 1 < p < 100000 and p is guaranteed to be a prime.   Output You should output the answer modulo p.   Sample Input 2 1 2 5 2 1 5   Sample Output 3 3 Hint Hint For sample 1, squirrels will put no more than 2 beans in one tree. Since trees are different, we can label them as 1, 2 … and so on. The 3 ways are: put no beans, put 1 bean in tree 1 and put 2 beans in tree 1. For sample 2, the 3 ways are: put no beans, put 1 bean in tree 1 and put 1 bean in tree 2.

求解C(n+m, m) % p。

AC代码：

#include <cstdio>
#include <cstring>
#include <algorithm>
#define MAXN 100005
#define LL long long
using namespace std;
LL fac[MAXN];
void getfac(LL p)
{
    fac[0] = 1 % p;
    for(LL i = 1; i <= p; i++)
        fac[i] = fac[i-1] * i % p;
}
LL pow_mod(LL a, LL n, LL p)
{
    LL ans = 1 % p;
    while(n)
    {
        if(n & 1)
            ans = ans * a % p;
        a = a * a % p;
        n >>= 1;
    }
    return ans;
}
LL C(LL n, LL m, LL p)
{
    if(m > n) return 0;
    else return fac[n] * pow_mod(fac[m] * fac[n-m] % p, p-2, p) % p;
}
LL Lucas(LL n, LL m, LL p)
{
    if(m == 0)
        return 1 % p;
    else
        return C(n%p, m%p, p) * Lucas(n/p, m/p, p) % p;
}
int main()
{
    int t;
    scanf("%d", &t);
    while(t--)
    {
        LL n, m, p;
        scanf("%lld%lld%lld", &n, &m, &p);
        getfac(p);
        printf("%lld\n", Lucas(n+m, m, p));
    }
    return 0;
}