字符串的排列

题目描述：

输入一个字符串,按字典序打印出该字符串中字符的所有排列。例如输入字符串abc,则打印出由字符a,b,c所能排列出来的所有字符串abc,acb,bac,bca,cab和cba

import java.util.*;public class Solution {    public ArrayList<String> Permutation(String str) {        ArrayList<String> re = new ArrayList<String>();        if (str == null || str.length() == 0) {            return re;        }        HashSet<String> set = new HashSet<String>();        fun(set, str.toCharArray(), 0);        re.addAll(set);        Collections.sort(re);        return re;    }    void fun(HashSet<String> re, char[] str, int beginIndex) {        if (beginIndex == str.length) {            //找到了一个字符串            re.add(new String(str));            return;        }        for (int i = beginIndex; i < str.length; i++) {            swap(str, i, beginIndex);            fun(re, str, beginIndex + 1);            swap(str, i, beginIndex);        }    }    void swap(char[] str, int i, int j) {        if (i != j) {            char t = str[i];            str[i] = str[j];            str[j] = t;        }    }}