求一个字符串的全排列
求输出一个字符串的全排列的程序？

例如输入“ab”，输出：“ab”“ba”
输入“abc”输出：“abc”“acb”“bac”“bca”“cab”“cba”,

设输入的字符串是“hello_world”

public class Main{        public static void main(String[] args) {        System.out.println(permutation("ab").length);        System.out.println(Arrays.toString(permutation("ab")));    }        private static String[] permutation(String orginal){        ArrayList list = new ArrayList();        if(orginal.length() == 1){            return new String[]{orginal};        } else{            for (int i = 0; i < orginal.length(); i++) {                String s = orginal.charAt(i) + "";                String result = "";                String resultA = result + s;                String leftS = orginal.substring(0, i) + orginal.substring(i + 1, orginal.length());                for (String element : permutation(leftS)) {                    result = resultA + element;                    list.add(result);                }            }            return (String[]) list.toArray(new String[list.size()]);        }    }    }