First, look at the subject
1, although I hate to say this, but I do not, but I mean you can not, but that does not always supposed to?
Second, look at problem solution
1, you have to know about this thing is what the test (though this thing ahead of you to say that d recursion), but certainly not the exam give you tips, you have to see it, in fact, recursive questions or good looking out.
2, to see if it is a recursive problem, so far as I understand, the general core are Fibonacci number, you see this problem and to tell the truth on Fibonacci like, it is to change a little test you nothing.
3, more important is to push equation, initial concluded that it is important, at the beginning of the first year, then I think it is have two cows, but it all wrong. We must look at the data from the sample.
4, anyway, the final formula is f [n] = f [n-1] + f [n-3];