do x^y in log(n)

/* Iterative Function to calculate (x^y) in O(log y) */
int power(int x, unsigned int y)
{
    int res = 1;
    while (y > 0)
    {
        // If y is odd, multiply x with result
        if (y & 1)
            res = res*x;

        // y must be even now
        y = y>>1; // y = y/2
        x = x*x; // Change x to x^2
    }
    return res;
}