Assign a String variable and made it static to fix the compilation error

src/main/java/com/github/hcsp/Main.java

@@ -1,7 +1,8 @@
 package com.github.hcsp;
 
 public class Main {
-    public static void main(String[] args) {
+    public static String sign;
+    public static void main(String[] args) {   // 程序开始
         System.out.println(relation(1, 2));
         System.out.println(relation(2, 1));
         System.out.println(relation(2, 2));
@@ -12,12 +13,13 @@ public static void main(String[] args) {
     // 修复编译错误
     // 在a<b时返回字符串"a<b"，在a>b时返回字符串"a>b"，否则返回"a=b"
     public static String relation(int a, int b) {
+
         if (a > b) {
-            String sign = ">";
+            sign = ">";
         } else if (a < b) {
-            String sign = "<";
+            sign = "<";
         } else {
-            String sign = "=";
+            sign = "=";
         }
 
         return "" + a + sign + b;