面试题：字符串的全排列

import java.util.List;
import java.util.Collections;
import java.util.ArrayList;
public class Solution {
    public ArrayList<String> Permutation(String str) {
        ArrayList<String> res=new ArrayList<>();
        if (str==null||str.length()==0) return res;
        PermutationHelper(str.toCharArray(),0,res);
        Collections.sort(res);
        return res;
    }
    public void PermutationHelper(char[] cs,int i,ArrayList<String> list){
        if(i==cs.length-1){//不是太懂这一步
            String val=String.valueOf(cs);
            if (!list.contains(val))
                list.add(val);
        }else{
            for (int j=i;j<cs.length;j++){
                swap(cs,i,j);
                PermutationHelper(cs,i+1,list);
                swap(cs,i,j);
            }
        }
    }
    public void swap(char[] cs,int i,int j){
        char temp =cs[i];
        cs[i]=cs[j];
        cs[j]=temp;
    }
}