求乘法逆元

#include <cstdio>
#include <cstring>
#include <cstdlib>
typedef long long ll;
using namespace std;
inline void (ll a, ll b, ll &d, ll &x, ll &y)
{
 if (!b) d = a, x = 1, y = 0;
    else exgcd(b, a % b, d, y, x), y -= x * (a / b);
}
inline ll inv(ll a, ll p)
{
  ll d, x, y;
  exgcd(a, p, d, x, y);
    return d == 1 ? (x + p) % p : -1;
}
int main(int argc, char **argv)
{
 ll a, b;
 cin >> a >> b;
 cout << inv(a, b) << endl;
   return 0;
}

#include <cstdio>
using namespace std;

const int p = 1e9 + 7;

int power(int a, int b)
{
 int ans = 1;
    for (; b; (a *= a) %= p, b >>= 1)
     if (b & 1) (ans *= a) %= p;
 return ans % p;
}

int main()
{
 int a;
  scanf("%d", &a);
   printf("%dn", power(a, p - 2));
}