优美的排列 leetcode 526 看不懂大神的代码，求解释

class Solution(object):
    def countArrangement(self, N):
        """
        :type N: int
        :rtype: int
        """
        def count(i, X, cache={}):
            if i < 2: return 1
            if X not in cache:               # X: set of still avaliable numbers
                cache[X] = sum(count(i-1, X-{x}) for x in X
                              if x%i == 0 or i % x == 0)
            return cache[X]
        return count(N, frozenset(range(1, N + 1))) # use frozenset to ensure the fixed order

https://blog.csdn.net/qq_38595487/article/details/81749766