In this post, we return back to the combinatorial problem discussed in Introduction to Dynamic Programming and Memoization post. We will show that generating functions may work great not only for single variable case (see The Art of Generating Functions), but also could be very useful for hacking two-variable relations (and of course, in general for multivariate case too).
For making the post self-contained, we repeat the problem definition here.

The notion of generating functions and its application to solving recursive equations are very well-known. For reader who did not have a chance to get familiar with the topics, I recommend to take a look at very good book: Concrete Mathematics: A Foundation for Computer Science, by Ronald L. Graham, Donald E. Knuth, Oren Patashnik.
Generating functions are usually applied to single variable recursive equations. But actually, the technique may be extended to multivariate recursive equations, or even to a system of recursive equations.

In the post, we discuss the basics of Recursion, Dynamic Programming (DP), and Memoization. As an example, we take a combinatorial problem, which has very short and clear description. This allows us to focus on DP and memoization. Note that the topics are very popular in coding interviews. Hopefully, this article will help to somebody to prepare for such types of questions.
In the next posts, we consider more advanced topics, like The Art of Generating Functions and Cracking Multivariate Recursive Equations Using Generating Functions.

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