P4071 [SDOI2016] 排列计数

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int mod = 1e9 + 7, N = 1e6 + 10;
int f[N], d[N];
int qpow(int b, int e) {
    int a = 1ll;
    while (e) {
        if (e & 1) a = a * b % mod;
        b = b * b % mod;
        e >>= 1;
    }
    return a;
}
signed main() {
    f[0] = d[0] = 1;
    for (int i = 1; i <= N; i ++ )
        f[i] = 1ll * f[i - 1] * i % mod,
        d[i] = (1ll * d[i - 1] * i + (i % 2 ? mod - 1 : 1)) % mod;
    int t;
    scanf("%lld", &t);
    while (t -- ) {
        int n, m;
        scanf("%lld%lld", &n, &m);
        int C = 1ll * f[n] * qpow(f[m], mod - 2) % mod * qpow(f[n - m], mod - 2) % mod;
        printf("%lld\n", 1ll * C * d[n - m] % mod);
    }
}