剑指offer-字符串的排序

import java.util.ArrayList;
import java.util.Collections;
public class Solution {
    ArrayList<String> re ;
    public ArrayList<String> Permutation(String str) {
       re = new ArrayList<String>();
       if(str==null||str.length()==0){
           return re;
       }
       char[] ch= str.toCharArray();
       help(ch,0,ch.length-1);
       Collections.sort(re);
       return re;
    }
    public void help(char[] ch,int left,int right){
        if(left ==right){
            re.add(String.valueOf(ch));
            return;
        }
        for(int i = left;i<=right;i++){
            if(i==left||ch[i]!=ch[left]){//字符可能重复
                swap(ch,left,i);
                help(ch,left+1,right);
                swap(ch,i,left);
            }
        }
    }
    private void swap(char[] s,int i,int j){
        char t = s[i];
        s[i] =s[j];
        s[j] =t;
    }
}