a^b_mod_p.cpp

const int mod = 1e9 + 7;

int power(int a, int b, int p = mod) {
  int ret = 1;
  a %= p;
  while (b > 0) {
    if (b % 2 == 1)
      ret = ret * a % p;
    b /= 2;
    a = a * a % p;
  }
  return ret;
}