Description: 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".
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File list (Check if you may need any files):
求子集.cpp