add recursion version to 206

Author: Li Li

Algorithms Basics/Chapter 2. Linked List/206. Reverse Linked List.cs

@@ -1,3 +1,4 @@
+// iteration
 public class Solution {
     public ListNode ReverseList(ListNode head) {
         ListNode pre = null, temp = null;
@@ -9,4 +10,15 @@ public ListNode ReverseList(ListNode head) {
         }
         return pre;
     }
+}
+
+// recursion
+public class Solution {
+    public ListNode ReverseList(ListNode head) {
+        if (head == null || head.next == null) return head;
+        ListNode newHead = ReverseList(head.next);
+        head.next.next = head;
+        head.next = null;
+        return newHead;
+    }
 }