Merge pull request #828 from casnz1601/patch-7

Update 0669.修剪二叉搜索树.md

problems/0669.修剪二叉搜索树.md

@@ -264,20 +264,37 @@ class Solution {
 
 ```
 
-## Python 
-
+## Python  
+**递归**
 ```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 trimBST(self, root: TreeNode, low: int, high: int) -> TreeNode:
-        if not root: return root
-        if root.val < low: 
-            return self.trimBST(root.right,low,high)  // 寻找符合区间[low, high]的节点
-        if root.val > high:
-            return self.trimBST(root.left,low,high)  // 寻找符合区间[low, high]的节点
-        root.left = self.trimBST(root.left,low,high)  // root->left接入符合条件的左孩子
-        root.right = self.trimBST(root.right,low,high)   // root->right接入符合条件的右孩子
-        return root
+        '''
+        确认递归函数参数以及返回值：返回更新后剪枝后的当前root节点
+        '''
+        # Base Case
+        if not root: return None
+
+        # 单层递归逻辑
+        if root.val < low:
+            # 若当前root节点小于左界：只考虑其右子树，用于替代更新后的其本身，抛弃其左子树整体
+            return self.trimBST(root.right, low, high)
+        
+        if high < root.val:
+            # 若当前root节点大于右界：只考虑其左子树，用于替代更新后的其本身，抛弃其右子树整体
+            return self.trimBST(root.left, low, high)
+
+        if low <= root.val <= high:
+            root.left = self.trimBST(root.left, low, high)
+            root.right = self.trimBST(root.right, low, high)
+            # 返回更新后的剪枝过的当前节点root
+            return root
 ```
 
 ## Go