Massive Algorithms: Buttercola: Zenefits: Combinations

Buttercola: Zenefits: Combinations
找出一个string里面长度为n （n小于等于string的长度）的所有组合。
这个比较简单考虑了一下dupilcate 字母

    public List<String> combination(String s, int k) {

        List<String> result = new ArrayList<>();

        if (s == null || s.length() == 0 || k <= 0) {

            return result;

}

        StringBuffer sb = new StringBuffer();

        combinationHelper(0, s, 0, k, sb, result);

        return result;

}

    private void combinationHelper(int start, String s, int num, 

                                   int k, StringBuffer sb, List<String> result) {

        if (num == k) {

            result.add(sb.toString());

            return;

}

        for (int i = start; i < s.length(); i++) {

            sb.append(s.charAt(i));

            combinationHelper(i + 1, s, num + 1, k, sb, result);

            sb.deleteCharAt(sb.length() - 1);

}

}

    public static void main(String[] args) {

        Solution solution = new Solution();

        String s = "abcd";

        List<String> result = solution.combination(s, 2);

        for (String i : result) {

            System.out.println(i);

}

}

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