What is the time complexity of this Factorization program? input: n-bit integer X

output: list of prime factors and their multiplicity

acc = X

for i = 2 ... sqrt( X )

--count = 0

--while acc % i == 0

------acc = acc / i

------count = count + 1

--if count &gt; 0

------print i (count)

I know that for the worst case, a n-bit integer is 2^n maximum.

So the time complexity for for i = 2 ... sqrt( X ) is 2^(n/2)-1

Now I'm really confused with the time complexity of while acc % i == 0

For the worst case, let's say that the n-bit number X is a prime. Then the while loop will not be executed at all. How can I define its time-complexity?

How can I find the Big-O of this algorithm? Is iit O(2^n) or O(2^(n/2))
Your algorithm takes $O(X^{1/2})$ in the worst case, which isn't too bad, but it also takes $\Theta(X^{1/2})$ in the best case. That's because you always test divisibility by all numbers up to $X^{1/2}$ even when it is clear that you found all factors of X already. 

The loop testing acc % i = 0 is negligible, because X has at most $\log X$ factors.