Because Fibonacci numbers are quite abused in programming, a similar concept. My first impulse is to describe them in recursive way: But this is not very efficient to calculate them, as for each is calculating all the previous ones, recursively. Here memoization works beautifully Taking into account that it uses more memory, and that calculating the Nth element without calculating the previous ones is also costly. I saw this on Programming Praxis, and I like a lot the solution proposed by Graham on the comments, using an iterator. The code is really clean.