组合数模板O(nlogn)预处理, O(1)出答案

typedef long long LL;
const int N = 5e5 + 10;
const LL mod = 1e9 + 7;
LL A[N], B[N];

void Init()
{
    A[0] = 1;
    for(int i = 1; i < N; i++){
        A[i] = A[i - 1] * i % mod;
    }
}

LL Ext_Gcd(LL a, LL b, LL &x, LL &y)
{
    if(!b){
        x = 1; y = 0;
        return a;
    }
    LL d = Ext_Gcd(b, a % b, y, x);
    y -= a / b * x;
    return d;
}

LL Inv(LL a, LL n)
{
    LL x, y;
    LL d = Ext_Gcd(a, n, x, y);
    if(d == 1)return (x % n + n) % n;
    return -1;
}

LL get()
{
    for(int i = 0;i < N; i++)
        B[i] = Inv(A[i], mod);
} 

LL C(LL a, LL b, LL mod)
{
    if(a < b)return 0;
    return (A[a] * B[b] % mod) * B[a - b] % mod;
}