python/TangDou/AcWing/Combinations/886.cpp

#include <bits/stdc++.h>

using namespace std;
#define int long long
#define endl "\n"
const int N = 100010;
const int p = 1e9 + 7;

int fact[N];   // 阶乘结果数组
int infact[N]; // 阶乘逆元结果数组

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

signed main() {
    // base case
    fact[0] = infact[0] = 1;

    for (int i = 1; i < N; i++) {
        fact[i] = fact[i - 1] * i % p;
        infact[i] = infact[i - 1] * qmi(i, p - 2) % p;
    }

    int n;
    cin >> n;
    while (n--) {
        int a, b;
        cin >> a >> b;
        printf("%lld\n", fact[a] * infact[b] % p * infact[a - b] % p);
    }
}