If n is small, use trial division.
A simple algorithm that sometimes works for larger integers is the
Pollard p - 1 algorithm from 1974 (see page 150). Input: odd
number n, and bound B:

Pollard p - 1 factoring algorithm (n; B)
	b := 2
	for j := 2 to B do
		b := bj mod n
	//b congruent 2B! (mod n))
	d := gcd(b - 1; n)
	if 1 < d < n then
		return d
	else
		return "failure"