瞎JB逆

P为质

 const int mod = ;
 long long quickpow(long long a, long long b) {
     if (b < ) return ;
     long long ret = ;
     a %= mod;
     while(b) {
         if (b & ) ret = (ret * a) % mod;
         b >>= ;
         a = (a * a) % mod;
     }
     return ret;
 }
 long long inv(long long a) {
     return quickpow(a, mod - );
 }  

N，P互质

 ll extend_gcd(ll a, ll b, ll &x, ll &y) {
     if (b == ) {
         x = , y = ;
         return a;
     }
     else {
         ll r = extend_gcd(b, a % b, y, x);
         y -= x * (a / b);
         return r;
     }
 }
 ll inv(ll a, ll n) {
     ll x, y;
     extend_gcd(a, n, x, y);
     x = (x % n + n) % n;
     return x;
 }  

逆元筛

 const int mod = ;
 const int maxn = ;
 int inv[maxn];
 inv[] = ;
 for(int i = ; i < ; i++)
     inv[i] = inv[mod % i] * (mod - mod / i) % mod;  

阶乘逆元筛(阶乘数组：fac[ ])

 fac[]=fac[]=;

 for(int i=;i<=MAXN;i++)fac[i]=fac[i-]*i%mod;

 inv[MAXN]=quipow(fac[MAXN],mod-);

 for(int i=MAXN-;i>=;i--)inv[i]=inv[i+]*(i+)%mod;