1. 下載壓縮包，解壓得到Reverse.class，用IDEA開啟，得到以下原始碼

 1 import java.util.ArrayList;
 2 import java.util.Scanner;
 3 
 4 public class Reverse {
 5     public Reverse() {
 6     }
 7 
 8     public static void main(String[] args) {
 9         Scanner s = new Scanner(System.in);
10         System.out.println("Please input the flag ：");
 
11         String str = s.next();
12         System.out.println("Your input is ：");
13         System.out.println(str);
14         char[] stringArr = str.toCharArray();
15         Encrypt(stringArr);
16     }
17 
18     public static void Encrypt(char[] arr) {
19         ArrayList<Integer> Resultlist = new
 
 ArrayList();
20 
21         for(int i = 0; i < arr.length; ++i) {
22             int result = arr[i] + 64 ^ 32;
23             Resultlist.add(result);
24         }
25 
26         int[] KEY = new int[]{180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65};
27         ArrayList<Integer> KEYList = new
 
 ArrayList();
28 
29         for(int j = 0; j < KEY.length; ++j) {
30             KEYList.add(KEY[j]);
31         }
32 
33         System.out.println("Result:");
34         if (Resultlist.equals(KEYList)) {
35             System.out.println("Congratulations！");
36         } else {
37             System.err.println("Error！");
38         }
39 
40     }
41 }

2. 大致的意思是：輸入的字串flag會被轉成字串列表，然後對每個元素進行加密。加密的方法是：每個元素加上64後再與32異或。加密完轉換成整型需與KEY各元素相等。所以對KEY中各元素異或32後減去64再轉char即可得到flag。

3. 解密程式碼如下：

 1 public class JavaReverseDecypto {
 2     public static void main(String[] args) {
 3         int[] KEY = new int[]{180, 136, 137, 147, 191, 137, 147, 191, 148, 136, 133, 191, 134, 140, 129, 135, 191, 65};
 4         int len = KEY.length;
 5 
 6         char[] flag = new char[len];
 7         for (int i = 0; i < len; ++i) {
 8             flag[i] = (char) ((KEY[i] ^ 32) - 64);
 9             System.out.print(flag[i]);
10         }
11     }
12 }

4. 輸出結果為This_is_the_flag_!