fast_pow_n.cpp

template<typename T>
T pow_n(T x, int n){
  T a = 1.0;
  
  while(n > 0){
    if(n % 2 == 0){
      x = x*x;
      n = n>>1;
    }else{
      a = a*x;
      n--;
    }
  }
  return a;
}

fastest calculation power