题目：
解法：
Set<List<Character>> getAllSubSet(String str, int digits) { // check the input if (str.matches("[^0-9]") || digits > str.length()) {throw new IllegalArgumentException("Invalid input");} // put every subSet into result Set<List<Character>> result = new HashSet<>(); for (int i = 0; i < (str.length() - digits + 1); i++) { List<Character> subSet = new ArrayList<>(); for (int j = 0; j < digits; j++) { subSet.add(str.charAt(i + j)); } result.add(subSet); } return result; }