Update linked_list.md

add a recursive method in java code to reverse a linked list.

Author: endaye

zh-hans/basics_data_structure/linked_list.md

@@ -96,6 +96,7 @@ class ListNode {
     }
 }
 
+// iterative method 
 public ListNode reverse(ListNode head) {
     ListNode prev = null;
     while (head != null) {
@@ -106,6 +107,18 @@ public ListNode reverse(ListNode head) {
     }
     return prev;
 }
+
+// recursive method 
+public ListNode reverse(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+        ListNode next = head.next;
+        ListNode newHead = reverse(next);
+        next.next = head;
+        head.next = null;
+        return newHead;
+    }
 ```
 
 #### 双向链表