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}。

输入：input.txt，仅包含整数n（1－26）。

输出：若输入合法，输出集合的所有子集；否则输出“WRONG”。子集输出格式为每行一个子集，空集用空行表示，非空集合每个元素间用一个空格间隔，最后一个元素之后不能有空格。-Write a recursive function of n a subset of all elements of the collection. May wish to make a collection of elements to lowercase, the original set is {　a　,　 b　, ...,　 a　+ n- 1}. Input: input.txt, contains only integer n (1-26). Output: If you enter a valid, all subsets of the output set　otherwise output　 WRONG　. A subset of the output format for each line of a subset of the empty set with a blank line to indicate the non-empty set with a space between each element spacing, no spaces after the last element.