Algorithm that converts numeric amount into English words

Question

What is the most efficient way to convert numeric amount into English words

e.g. 12 to twelve 127 to one hundred twenty-seven

Answer 1

.) make a library of all numbers & positions (e.g. 1 has other notation than 10, another than 100 etc) .) make a list of exceptions (e.g. for 12) and be aware, that in your algorythm, the same exception are for 112, 1012 etc.

if you want even more speed, make a cached set of numbers that you need.