进阶 Day 3
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#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
const int N = 100010, MOD = 1e9 + 7;
int n;
int fact[N], infact[N];
int qmi(int a, int k, int b)
{
int res = 1;
while (k)
{
if (k & 1)
res = (LL)res * a % MOD;
k >>= 1;
a = (LL)a * a % MOD;
}
return res;
} //qmi
int main(void)
{
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;
}
scanf("%d", &n);
while (n--)
{
int a, b;
scanf("%d%d", &a, &b);
int res = (LL)fact[a] * infact[a - b] % MOD * infact[b] % MOD;
printf("%d\n", res);
}
return 0;
}