题目链接：求组合数 II

#include <iostream>
#include <algorithm>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;
}