gold-8-7

无重复字符串的排列组合。编写一种方法，计算某字符串的所有排列组合，字符串每个字符均不相同。

示例1:

输入：S = "qwe"
输出：["qwe", "qew", "wqe", "weq", "ewq", "eqw"]

示例2:

输入：S = "ab"
输出：["ab", "ba"]

提示:

1. 字符都是英文字母。
2. 字符串长度在[1, 9]之间。

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public String[] permutation(String S) {
        if (S == null || S.length() == 0) {
            return new String[0];
        }

        List<String> result = new ArrayList<>();
        doPermute(result, "", S);
        return result.toArray(new String[result.size()]);
    }

    private void doPermute(List<String> result, String curS, String S) {
        if (curS.length() == S.length()) {
            result.add(new String(curS));
            return;
        }

        for (char ch : S.toCharArray()) {
            if (curS.indexOf(ch) == -1) {
                doPermute(result, curS + ch, S);
            }
        }
    }
}
//leetcode submit region end(Prohibit modification and deletion)