幂取模

 1 typedef long long ll;
 2 #define MOD 1000000007
 3 ll pow_mod(ll a, ll n)
 4 {
 5     if(n == 0)  return 1;
 6     ll x = pow_mod(a, n/2);
 7     ll ans = x*x%MOD;
 8     if(n%2==1)  ans = ans*a%MOD;
 9     return ans;
10 }