首页上题目难度的排序

template/src/oj/problem/problem_list.html

@@ -21,8 +21,8 @@
                             <th></th>
                             <th>#</th>
                             <th>题目</th>
-                            <th><a href="/problems/?order_by=difficulty">难度</a></th>
-                            <th><a href="/problems/?order_by=acceptance">通过率</a></th>
+                            <th><a href="/problems/?order_by={{ difficulty_order }}">难度</a></th>
+                            <th>通过率</th>
                         </tr>
                         </thead>
                         <tbody>