From b38880b2fdd6f99312811e759a25082be0c48ef0 Mon Sep 17 00:00:00 2001
From: Asterisk <44215173+casnz1601@users.noreply.github.com>
Date: Sun, 10 Oct 2021 12:26:42 +0800
Subject: [PATCH] =?UTF-8?q?Update=200538.=E6=8A=8A=E4=BA=8C=E5=8F=89?=
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补充python注释
---
 ...38.把二叉搜索树转换为累加树.md | 42 ++++++++++++++-----
 1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/problems/0538.把二叉搜索树转换为累加树.md b/problems/0538.把二叉搜索树转换为累加树.md
index f2b97989..6de98c6f 100644
--- a/problems/0538.把二叉搜索树转换为累加树.md
+++ b/problems/0538.把二叉搜索树转换为累加树.md
@@ -196,20 +196,40 @@ class Solution {
 ```
 
 ## Python 
+**递归**
 
-递归法
-```python
+```python3
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
 class Solution:
-    def convertBST(self, root: TreeNode) -> TreeNode:
-        def buildalist(root): 
-            if not root: return None
-            buildalist(root.right)  #右中左遍历
-            root.val += self.pre
-            self.pre = root.val
-            buildalist(root.left)
-        self.pre = 0  #记录前一个节点的数值
-        buildalist(root)
+    def __init__(self):
+        self.pre = TreeNode()
+
+    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        '''
+        倒序累加替换：  
+        [2, 5, 13] -> [[2]+[1]+[0], [2]+[1], [2]] -> [20, 18, 13]
+        '''
+        self.traversal(root)
         return root
+
+    def traversal(self, root: TreeNode) -> None:
+        # 因为要遍历整棵树，所以递归函数不需要返回值
+        # Base Case
+        if not root: 
+            return None
+        # 单层递归逻辑：中序遍历的反译 - 右中左
+        self.traversal(root.right)  # 右
+
+        # 中节点：用当前root的值加上pre的值
+        root.val += self.pre.val    # 中
+        self.pre = root             
+
+        self.traversal(root.left)   # 左
 ```
 
 ## Go