Update 二叉树相关.md

blogs/Algorithm/剑指 Offer/二叉树相关.md

@@ -14,6 +14,9 @@
 8. [32 - II. 从上到下打印二叉树 II](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/)
 9. [32 - III. 从上到下打印二叉树 III](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/)
 10. [37. 序列化二叉树](https://leetcode-cn.com/problems/xu-lie-hua-er-cha-shu-lcof/)
+11. [28. 对称的二叉树](https://leetcode-cn.com/problems/dui-cheng-de-er-cha-shu-lcof/)
+12. [33. 二叉搜索树的后序遍历序列](https://leetcode-cn.com/problems/er-cha-sou-suo-shu-de-hou-xu-bian-li-xu-lie-lcof/)
+13. [Offer 36. 二叉搜索树与双向链表](https://leetcode-cn.com/problems/er-cha-sou-suo-shu-yu-shuang-xiang-lian-biao-lcof/)
 
 #### [07. 重建二叉树](https://leetcode-cn.com/problems/zhong-jian-er-cha-shu-lcof/)
 
@@ -510,3 +513,42 @@ class Solution {
 }
 ```
 
+#### [Offer 36. 二叉搜索树与双向链表](https://leetcode-cn.com/problems/er-cha-sou-suo-shu-yu-shuang-xiang-lian-biao-lcof/)
+
+```java
+class Solution {
+
+    public Node treeToDoublyList(Node root) {
+        if (root == null) {
+            return root;
+        }
+        List<Node> list = new ArrayList<>();
+        helper(list, root);
+        Node head = list.get(0);
+        Node pre = head;
+        for (int i = 1; i < list.size(); i++) {
+            Node node = list.get(i);
+            pre.right = node;
+            node.left = pre;
+            pre = pre.right;
+        }
+        pre.right = head;
+        head.left = pre;
+        return head;
+    }
+
+    private void helper(List<Node> list, Node root) {
+        if (root == null) {
+            return;
+        }
+        if (root.left != null) {
+            helper(list, root.left);
+        }
+        list.add(root);
+        if (root.right != null) {
+            helper(list, root.right);
+        }
+    }
+}
+```
+