when-greatest-is-less-than-smallest

When greatest is less than smallest

Given an x and y find the smallest and greatest numbers above and below a given n that are divisible by both x and y.

Examples

GreatestSmallest.greatest(new BigInteger("2"), new BigInteger("3"), new BigInteger("20")) => BigInteger("18") // 18 is the greatest number under 20 that is divisible by both 2 and 3
GreatestSmallest.greatest(new BigInteger("2"), new BigInteger("3"), new BigInteger("20")) => BigInteger("24") // 24 is the smallest number above 20 that is divisible by both 2 and 3

GreatestSmallest.greatest(new BigInteger("5"), new BigInteger("15"), new BigInteger("100")) => BigInteger("90")
GreatestSmallest.smallest(new BigInteger("5"), new BigInteger("15"), new BigInteger("100")) => BigInteger("105")

GreatestSmallest.greatest(new BigInteger("123"), new BigInteger("456"), new BigInteger("789")) => BigInteger("0") // there are no numbers under 789 that are divisible by both 123 and 456
GreatestSmallest.smallest(new BigInteger("123"), new BigInteger("456"), new BigInteger("789")) => BigInteger("18696")

Notes

1. you should never return n even if it is divisible by x and y always the number above or below it
2. greatest should return 0 if there are no numbers under n that are divisible by both x and y
3. and all arguments will be valid (integers greater than 0).