2023-9-8 求组合数(二)

题目链接：求组合数 II

#include 
#include 

using namespace std;


typedef long long LL;
const int mod = 1e9 + 7;
const int N = 100010;

// 阶乘，阶乘的逆
int fact[N], infact[N];


LL qmi(int a, int k, int p)
{
    int res = 1;
    while(k)
    {
        if(k & 1) res = (LL) res * a % p;
        a = (LL) a * a % p;
        k >>= 1;
    }
    return res;
}

int main()
{
    
    fact[0] = infact[0] = 1;
    for(int i = 1; i < N; i++)
    {
        fact[i] = (LL)fact[i - 1] * i % mod;
        infact[i] = (LL)infact[i - 1] * qmi(i, mod - 2, mod) % mod;
    }
    
    int n;
    cin >> n;
    while(n--)
    {
        int a, b;
        cin >> a >> b;
        cout << (LL) fact[a] * infact[b] % mod * infact[a - b] % mod << endl;
    }
    
    return 0;
}
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