Merge pull request #89 from Joshua-Lu/patch-5

添加 0206.翻转链表 Java版本

problems/0206.翻转链表.md

@@ -103,24 +103,44 @@ public:
 
 Java：
 ```java
+// 双指针
 class Solution {
     public ListNode reverseList(ListNode head) {
-        if (head == null || head.next == null) return head;
+        ListNode prev = null;
-        ListNode last = head;
+        ListNode cur = head;
-        ListNode cur = head.next;
+        ListNode temp = null;
-        head.next = null;
+        while (cur != null) {
-        while (cur.next != null) {
+            temp = cur.next;// 保存下一个节点
-            ListNode tmp = cur.next;
+            cur.next = prev;
-            cur.next = last;
+            prev = cur;
-            last = cur;
+            cur = temp;
-            cur = tmp;
         }
-        cur.next = last;
+        return prev;
-        return cur;
     }
 }
 ```
 
+```java
+// 递归 
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        return reverse(null, head);
+    }
+
+    private ListNode reverse(ListNode prev, ListNode cur) {
+        if (cur == null) {
+            return prev;
+        }
+        ListNode temp = null;
+        temp = cur.next;// 先保存下一个节点
+        cur.next = prev;// 反转
+        // 更新prev、cur位置
+        prev = cur;
+        cur = temp;
+        return reverse(prev, cur);
+    }
+}
+```
 
 Python：