Question

Interview question:

Calculate x ^ y in O(log n)

There are different answers like “Use the Indian Power algorithm” or

double power(double x, int y) 
{
    if(y == 0) return 1;

    double d = power(x, y/2);

    if(y%2 == 0) return d*d;
    else return x*d*d;
}

1. is it a correct answer?

2. is there any better way of writing a code for this question?

Answer 1

This is called Exponentiation by Squaring. As far as implementation goes, it is a matter of taste. Your recursive implementation is good, but non-recursive implementations (from the link above) may look a little cleaner to people who do not like recursion or erroneously believe that it is somehow “wasteful” or “slow”.