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, an n-element set of all subset of. May wish to make a collection of elements to lowercase letters, the original set is {' a' , ' b' , ..., ' a' + n- 1}. Input: input.txt contains only the integer n (1-26). Output: enter a valid subset of the output collection　otherwise output " WRONG" . A subset of a subset of the output format for each line, empty set with an empty line, non-empty set with a space interval between each element of an element can not have spaces.