快速计算逆元

#include<bits/stdc++.h>#define ll long longusing namespace std;const int N= 1e6+100;const int mod = 1e9+7;ll f[N],invf[N],inv[N];void init(){    inv[1] =  1;    f[0] =  invf[0] = 1;    for(int i = 2;i < N;i ++){        inv[i] = (mod-mod/i)*1LL*inv[mod%i]%mod;    }    for(int i =1;i < N;i ++){        f[i] = f[i-1]*i%mod;    }    for(int i = 1;i < N;i ++){        invf[i] = invf[i-1]*1LL*inv[i]%mod;    }}