字符串的排列

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
*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
*
*输入一个字符串,长度不超过9(可能有字符重复),字符只包括大小写字母。按字典序打印出该字符串中字符的所有排列
*/
public class Solution {
    public List<String> permutation(String str) {
        if(str==null){
            return null;
        }
           char[] ary= str.toCharArray();
        Set<String> set = new HashSet<String>();
        List<Integer> flag = new ArrayList<Integer>();
        StringBuilder sbl=new StringBuilder();
        int index=0;
        this.handler(ary,sbl,set,flag,index);
        List<String> list = new ArrayList<String>();
        list.addAll(set);
        Collections.sort(list);
        return list;
    }
    public void handler(char[] ary ,StringBuilder sb,Set<String> set ,List<Integer> flag ,int index){
        for(int i = 0 ; i< ary.length ; i++){
            
            if(sb.length()>index){
                sb.delete(index,sb.length());
                while(index<flag.size()){
                    flag.remove(index);
                }
            }
            if(!flag.contains(i)){
                sb.append(ary[i]);
                flag.add(i);
                if(sb.length()<ary.length){
                    this.handler(ary,sb,set,flag,index+1);
                }
                if(sb.length()==ary.length){
                    set.add(sb.toString());
                }
            }
        }
    }
}