一个列举出所有数字的方法,该怎么处理

import java.util.*;public class Test{ public static void main(String[] args) {        System.out.println(Arrays.toString(permutation("1234")));    }        private static String[] permutation(String orginal){        ArrayList list = new ArrayList();        if(orginal.length() == 1){            return new String[]{orginal};        } else{           for (int i = 0; i < orginal.length(); i++) {             String s = orginal.charAt(i) + "";             String result = "";             String resultA = result + s;     String leftS = orginal.substring(0, i) + orginal.substring(i + 1, orginal.length());             for (String element : permutation(leftS)) {                    result = resultA + element;                     list.add(result);                }            }            return (String[]) list.toArray(new String[list.size()]);        }    }}