剑指offer_字符串---字符串的排列

import java.util.ArrayList;
import java.util.TreeSet;
public class Solution {
    public ArrayList<String> Permutation(String str) {
       ArrayList<String> list = new ArrayList<String>();
       if(str==null||str.length()<0){
           return list;
       }
       TreeSet<String> set = new TreeSet<String>();
       helper(str.toCharArray(),set,0);
       list.addAll(set);
        return list;
    }
    public void helper(char[] str,TreeSet<String> set,int cur){
        if(cur == str.length-1){
            set.add(String.valueOf(str));
            return;
        }
        for(int i = cur ; i<str.length; i++){
            exchange(str,i,cur);
            helper( str, set, cur+1);
            exchange(str,i,cur);
        }
    }
    public void exchange(char[] str,int i,int j){
        char temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }
}