集合划分，输入一个小于或等于15的正整数n，表示某个集合有n个元素，输出为这n个元素的集合能够划分成多少个由子集构成的集合-Set partitioning, enter a value less than or equal to 15 positive integer n, represents a collection with n elements, the output is a set of n elements which can be divided into a number o

编写递归函数，求n个元素集合的所有子集。不妨令集合元素为小写字母，原集合为{‘a’, ‘b’, …, ‘a’ + n - 1}。-Write a recursive function, all subsets of a set of elements for n