Problem: Suppose $$$cnt(i)$$$ represents the number of occurrences of $$$i$$$ in array $$$A$$$ of length $$$n$$$ whose elements are between $$$1$$$ and $$$n$$$. An array is called a $$$k$$$-good array if and only if $$$cnt(k)=k$$$. Let $$$f(k)$$$ be the number of $$$k$$$-good array. You are to calculate the $$$\sum\limits_{k=1}^n f(k)$$$.

This problem looks like some dp problems which can be reduced to a simper form and solved by Kunth's Mechanical Summation. But I haven't thought up a good solution.

Could you share your ideas about this problem?