Update 0538.把二叉搜索树转换为累加树.md

增加 python 迭代法

problems/0538.把二叉搜索树转换为累加树.md

@@ -234,6 +234,26 @@ class Solution:
         return root 
         
 ```
+**迭代**
+```python
+class Solution:
+    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root: return root
+        stack = []
+        result = []
+        cur = root
+        pre = 0
+        while cur or stack:
+            if cur:
+                stack.append(cur)
+                cur = cur.right
+            else: 
+                cur = stack.pop()
+                cur.val+= pre
+                pre = cur.val
+                cur =cur.left
+        return root
+```            
 
 ## Go