回溯法（Java）

思路：
回溯法 + set去重 + 集合排序

import java.util.*;

public class Solution {

    public ArrayList<String> Permutation(String str) {
        Set<String> list = new HashSet<>();
        backtrace(list,new StringBuilder(str),0);
        ArrayList result = new ArrayList<>(list);
        Collections.sort(result);
        return result;
    }



    public void backtrace(Set list,StringBuilder str,int index){
        if(index + 1 == str.length()){
            list.add(str.toString());
        }else{
            for(int i = index;i < str.length();i++){
                char temp = str.charAt(index);
                str.setCharAt(index,str.charAt(i));
                str.setCharAt(i,temp);
                backtrace(list,str,index + 1);
                temp = str.charAt(index);
                str.setCharAt(index,str.charAt(i));
                str.setCharAt(i,temp);
            }
        }
    }
}