字符串的排序 --剑指offer

import java.util.ArrayList;
import java.util.*;
public class Solution {
    public ArrayList<String> Permutation(String str) {
        ArrayList<String> list=new ArrayList<>();
        if (str!= null && str.length() > 0){
            PermutationHelper(str.toCharArray(),0,list);
            Collections.sort(list);
        }
        
        return  list;
    }

    public void PermutationHelper(char[] chars, int i, ArrayList<String> list) {
        if(i == chars.length -1){
            String str=String.valueOf(chars);
            if(!list.contains(str)){
                list.add(str);
            }
        }else {
                for(int j =i;j < chars.length;j ++){
                    swap(chars,i,j);
                    PermutationHelper(chars,i+1,list);
                    swap(chars,i,j);
                }
            }
        }

    public void swap(char[] chars, int i, int j) {
        char tem=chars[i];
        chars[i]=chars[j];
        chars[j] = tem;
    }
}