牛客网诡异的判题系统

代码

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
public class Solution {
    public ArrayList<String> Permutation(String str){
        if (str==null||str.length()==0){
            return  new ArrayList<>();
        }else {
            HashSet<String>  set=new HashSet<>();
            String  newstr="";
            PermutationHelper(set,newstr,str);
            ArrayList<String> arrayList=new ArrayList<>();
            Iterator<String>   iterator=set.iterator();
            while (iterator.hasNext()){
                arrayList.add(iterator.next());
            }
            return  arrayList;
        }
    }
    public  void  PermutationHelper(HashSet<String> set, String newstr, String  str){
        if (str.length()==1){
            newstr=newstr+str;
            set.add(newstr);
            return;
        }
        for (int i=0;i<str.length();i++){
            String  s= str.charAt(i)+"";
            String  str1 = "";
            String  str2 = "";
            if (i-1>-1&&i+1<str.length()){
                str1=str.substring(0,i);
                str2=str.substring(i+1,str.length());
            }else if (i-1<0){
                str1=str.substring(1,str.length());
            }else if (i+1>=str.length()){
                str1=str.substring(0,str.length()-1);
            }
            PermutationHelper(set,newstr+s,str1+str2);
        }
  }
}