diff --git a/problems/0098.验证二叉搜索树.md b/problems/0098.验证二叉搜索树.md
index ccb63305..95b657a5 100644
--- a/problems/0098.验证二叉搜索树.md
+++ b/problems/0098.验证二叉搜索树.md
@@ -371,117 +371,113 @@ class Solution {
 
 ## Python 
 
-**递归** - 利用BST中序遍历特性,把树"压缩"成数组
+递归法（版本一）利用中序递增性质，转换成数组
 ```python
+# 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 isValidBST(self, root: TreeNode) -> bool:
-        # 思路: 利用BST中序遍历的特性.
-        # 中序遍历输出的二叉搜索树节点的数值是有序序列
-        candidate_list = []
-        
-        def __traverse(root: TreeNode) -> None: 
-            nonlocal candidate_list
-            if not root: 
-                return 
-            __traverse(root.left)
-            candidate_list.append(root.val)
-            __traverse(root.right)
-            
-        def __is_sorted(nums: list) -> bool: 
-            for i in range(1, len(nums)): 
-                if nums[i] <= nums[i - 1]: # ⚠️ 注意: Leetcode定义二叉搜索树中不能有重复元素
-                    return False
-            return True
-        
-        __traverse(root)
-        res = __is_sorted(candidate_list)
-        
-        return res
-```
+    def __init__(self):
+        self.vec = []
 
-**递归** - 标准做法
+    def traversal(self, root):
+        if root is None:
+            return
+        self.traversal(root.left)
+        self.vec.append(root.val)  # 将二叉搜索树转换为有序数组
+        self.traversal(root.right)
 
-```python
-class Solution:
-    def isValidBST(self, root: TreeNode) -> bool:
-        # 规律: BST的中序遍历节点数值是从小到大. 
-        cur_max = -float("INF")
-        def __isValidBST(root: TreeNode) -> bool: 
-            nonlocal cur_max
-            
-            if not root: 
-                return True
-            
-            is_left_valid = __isValidBST(root.left)
-            if cur_max < root.val: 
-                cur_max = root.val
-            else: 
+    def isValidBST(self, root):
+        self.vec = []  # 清空数组
+        self.traversal(root)
+        for i in range(1, len(self.vec)):
+            # 注意要小于等于，搜索树里不能有相同元素
+            if self.vec[i] <= self.vec[i - 1]:
                 return False
-            is_right_valid = __isValidBST(root.right)
-            
-            return is_left_valid and is_right_valid
-        return __isValidBST(root)
+        return True
+
 ```
-**递归** - 避免初始化最小值做法：
+
+递归法（版本二）设定极小值，进行比较
+
 ```python
 class Solution:
-    def isValidBST(self, root: TreeNode) -> bool:
-        # 规律: BST的中序遍历节点数值是从小到大. 
-        pre = None
-        def __isValidBST(root: TreeNode) -> bool: 
-            nonlocal pre
-            
-            if not root: 
-                return True
-            
-            is_left_valid = __isValidBST(root.left)
-            if pre and pre.val>=root.val: return False
-            pre = root
-            is_right_valid = __isValidBST(root.right)
-            
-            return is_left_valid and is_right_valid
-        return __isValidBST(root)
+    def __init__(self):
+        self.maxVal = float('-inf')  # 因为后台测试数据中有int最小值
+
+    def isValidBST(self, root):
+        if root is None:
+            return True
+
+        left = self.isValidBST(root.left)
+        # 中序遍历，验证遍历的元素是不是从小到大
+        if self.maxVal < root.val:
+            self.maxVal = root.val
+        else:
+            return False
+        right = self.isValidBST(root.right)
+
+        return left and right
+
 ```
+递归法（版本三）直接取该树的最小值
 ```python
-迭代-中序遍历
+# 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 isValidBST(self, root: TreeNode) -> bool:
+    def __init__(self):
+        self.pre = None  # 用来记录前一个节点
+
+    def isValidBST(self, root):
+        if root is None:
+            return True
+
+        left = self.isValidBST(root.left)
+
+        if self.pre is not None and self.pre.val >= root.val:
+            return False
+        self.pre = root  # 记录前一个节点
+
+        right = self.isValidBST(root.right)
+        return left and right
+
+
+
+```
+迭代法
+```python
+# 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 isValidBST(self, root):
         stack = []
         cur = root
-        pre = None
-        while cur or stack:
-            if cur: # 指针来访问节点，访问到最底层
+        pre = None  # 记录前一个节点
+        while cur is not None or len(stack) > 0:
+            if cur is not None:
                 stack.append(cur)
-                cur = cur.left
-            else: # 逐一处理节点
-                cur = stack.pop()
-                if pre and cur.val <= pre.val: # 比较当前节点和前节点的值的大小
+                cur = cur.left  # 左
+            else:
+                cur = stack.pop()  # 中
+                if pre is not None and cur.val <= pre.val:
                     return False
-                pre = cur 
-                cur = cur.right
+                pre = cur  # 保存前一个访问的结点
+                cur = cur.right  # 右
         return True
 
 ```
-```python
-# 遵循Carl的写法，只添加了节点判断的部分
-class Solution:
-    def isValidBST(self, root: TreeNode) -> bool:
-        # method 2
-        que, pre = [], None
-        while root or que:
-            while root:
-                que.append(root)
-                root = root.left
-            root = que.pop()
-            # 对第一个节点只做记录，对后面的节点进行比较
-            if pre is None:
-                pre = root.val
-            else:
-                if pre >= root.val: return False
-                pre = root.val
-            root = root.right
-        return True
-```
+
 
 ## Go
 
diff --git a/problems/0108.将有序数组转换为二叉搜索树.md b/problems/0108.将有序数组转换为二叉搜索树.md
index 89778421..056ef3e2 100644
--- a/problems/0108.将有序数组转换为二叉搜索树.md
+++ b/problems/0108.将有序数组转换为二叉搜索树.md
@@ -316,73 +316,65 @@ class Solution {
 ```
 
 ## Python  
-**递归**
-
+递归法
 ```python
-# 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 sortedArrayToBST(self, nums: List[int]) -> TreeNode:
-        '''
-        构造二叉树：重点是选取数组最中间元素为分割点，左侧是递归左区间；右侧是递归右区间
-        必然是平衡树
-        左闭右闭区间
-        '''
-        # 返回根节点
-        root = self.traversal(nums, 0, len(nums)-1)
-        return root
-
     def traversal(self, nums: List[int], left: int, right: int) -> TreeNode:
-        # Base Case
         if left > right:
             return None
         
-        # 确定左右界的中心，防越界
         mid = left + (right - left) // 2
-        # 构建根节点
-        mid_root = TreeNode(nums[mid])
-        # 构建以左右界的中心为分割点的左右子树
-        mid_root.left = self.traversal(nums, left, mid-1)
-        mid_root.right = self.traversal(nums, mid+1, right)
+        root = TreeNode(nums[mid])
+        root.left = self.traversal(nums, left, mid - 1)
+        root.right = self.traversal(nums, mid + 1, right)
+        return root
+    
+    def sortedArrayToBST(self, nums: List[int]) -> TreeNode:
+        root = self.traversal(nums, 0, len(nums) - 1)
+        return root
 
-        # 返回由被传入的左右界定义的某子树的根节点
-        return mid_root
 ```
 
-**迭代**（左闭右开）
+迭代法
 ```python
+from collections import deque
+
 class Solution:
-    def sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:
-        if len(nums) == 0: return None
-        root = TreeNode()  # 初始化
-        nodeSt = [root]
-        leftSt = [0]
-        rightSt = [len(nums)]
-
-        while nodeSt:
-            node = nodeSt.pop()  # 处理根节点
-            left = leftSt.pop()
-            right = rightSt.pop()
-            mid = left + (right - left) // 2
-            node.val = nums[mid]
-
-            if left < mid:  # 处理左区间
-                node.left = TreeNode()
-                nodeSt.append(node.left)
-                leftSt.append(left)
-                rightSt.append(mid)
-            
-            if right > mid + 1:  # 处理右区间
-                node.right = TreeNode()
-                nodeSt.append(node.right)
-                leftSt.append(mid + 1)
-                rightSt.append(right)
+    def sortedArrayToBST(self, nums: List[int]) -> TreeNode:
+        if len(nums) == 0:
+            return None
         
+        root = TreeNode(0)  # 初始根节点
+        nodeQue = deque()   # 放遍历的节点
+        leftQue = deque()   # 保存左区间下标
+        rightQue = deque()  # 保存右区间下标
+        
+        nodeQue.append(root)               # 根节点入队列
+        leftQue.append(0)                  # 0为左区间下标初始位置
+        rightQue.append(len(nums) - 1)     # len(nums) - 1为右区间下标初始位置
+
+        while nodeQue:
+            curNode = nodeQue.popleft()
+            left = leftQue.popleft()
+            right = rightQue.popleft()
+            mid = left + (right - left) // 2
+
+            curNode.val = nums[mid]  # 将mid对应的元素给中间节点
+
+            if left <= mid - 1:  # 处理左区间
+                curNode.left = TreeNode(0)
+                nodeQue.append(curNode.left)
+                leftQue.append(left)
+                rightQue.append(mid - 1)
+
+            if right >= mid + 1:  # 处理右区间
+                curNode.right = TreeNode(0)
+                nodeQue.append(curNode.right)
+                leftQue.append(mid + 1)
+                rightQue.append(right)
+
         return root
+
 ```
 
 ## Go 
diff --git a/problems/0110.平衡二叉树.md b/problems/0110.平衡二叉树.md
index c0561e10..e10a612a 100644
--- a/problems/0110.平衡二叉树.md
+++ b/problems/0110.平衡二叉树.md
@@ -536,18 +536,16 @@ class Solution:
 
 ```python
 class Solution:
-    def isBalanced(self, root: TreeNode) -> bool:
-        return self.height(root) != -1
-    def height(self, node: TreeNode) -> int:
-            if not node:
-                return 0
-            left = self.height(node.left)
-            if left == -1:
-                return -1
-            right = self.height(node.right)
-            if right == -1 or abs(left - right) > 1:
-                return -1
-            return max(left, right) + 1
+    def isBalanced(self, root: Optional[TreeNode]) -> bool:
+        return self.get_hight(root) != -1
+    def get_hight(self, node):
+        if not node:
+            return 0
+        left = self.get_hight(node.left)
+        right = self.get_hight(node.right)
+        if left == -1 or right == -1 or abs(left - right) > 1:
+            return -1
+        return max(left, right) + 1
 ```
 
 
diff --git a/problems/0111.二叉树的最小深度.md b/problems/0111.二叉树的最小深度.md
index d0bdf4be..a1fc8a93 100644
--- a/problems/0111.二叉树的最小深度.md
+++ b/problems/0111.二叉树的最小深度.md
@@ -302,36 +302,72 @@ class Solution {
 
 ## Python 
 
-递归法：
+递归法（版本一）
 
 ```python
-# 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 minDepth(self, root: TreeNode) -> int:
-        if not root:
+    def getDepth(self, node):
+        if node is None:
             return 0
+        leftDepth = self.getDepth(node.left)  # 左
+        rightDepth = self.getDepth(node.right)  # 右
         
-        if not root.left and not root.right:
-            return 1
+        # 当一个左子树为空，右不为空，这时并不是最低点
+        if node.left is None and node.right is not None:
+            return 1 + rightDepth
         
-        left_depth = float('inf')
-        right_depth = float('inf')
+        # 当一个右子树为空，左不为空，这时并不是最低点
+        if node.left is not None and node.right is None:
+            return 1 + leftDepth
         
-        if root.left:
-            left_depth = self.minDepth(root.left)
-        if root.right:
-            right_depth = self.minDepth(root.right)
-        
-        return 1 + min(left_depth, right_depth)
+        result = 1 + min(leftDepth, rightDepth)
+        return result
+
+    def minDepth(self, root):
+        return self.getDepth(root)
 
 ```
+递归法（版本二）
 
-迭代法：
+```python
+class Solution:
+    def minDepth(self, root):
+        if root is None:
+            return 0
+        if root.left is None and root.right is not None:
+            return 1 + self.minDepth(root.right)
+        if root.left is not None and root.right is None:
+            return 1 + self.minDepth(root.left)
+        return 1 + min(self.minDepth(root.left), self.minDepth(root.right))
+
+
+```
+递归法（版本三）前序
+
+```python
+class Solution:
+    def __init__(self):
+        self.result = float('inf')
+
+    def getDepth(self, node, depth):
+        if node is None:
+            return
+        if node.left is None and node.right is None:
+            self.result = min(self.result, depth)
+        if node.left:
+            self.getDepth(node.left, depth + 1)
+        if node.right:
+            self.getDepth(node.right, depth + 1)
+
+    def minDepth(self, root):
+        if root is None:
+            return 0
+        self.getDepth(root, 1)
+        return self.result
+
+
+```
+迭代法
 
 ```python
 # Definition for a binary tree node.
@@ -364,39 +400,7 @@ class Solution:
         return depth
 ```
 
-迭代法：
 
-```python
-# 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 minDepth(self, root: TreeNode) -> int:
-        if not root:
-            return 0
-        
-        queue = collections.deque([(root, 1)])
-        
-        while queue:
-            node, depth = queue.popleft()
-            
-            # Check if the node is a leaf node
-            if not node.left and not node.right:
-                return depth
-            
-            # Add left and right child to the queue
-            if node.left:
-                queue.append((node.left, depth+1))
-            if node.right:
-                queue.append((node.right, depth+1))
-        
-        return 0
-
-```
 
 ## Go
 
diff --git a/problems/0235.二叉搜索树的最近公共祖先.md b/problems/0235.二叉搜索树的最近公共祖先.md
index 8353303a..9777bb0b 100644
--- a/problems/0235.二叉搜索树的最近公共祖先.md
+++ b/problems/0235.二叉搜索树的最近公共祖先.md
@@ -275,34 +275,57 @@ class Solution {
 
 ## Python
 
-递归法：
+递归法（版本一）
 ```python
 class Solution:
-    """二叉搜索树的最近公共祖先 递归法"""
+    def traversal(self, cur, p, q):
+        if cur is None:
+            return cur
+                                                        # 中
+        if cur.val > p.val and cur.val > q.val:           # 左
+            left = self.traversal(cur.left, p, q)
+            if left is not None:
+                return left
 
-    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
-        if root.val > p.val and root.val > q.val:
-            return self.lowestCommonAncestor(root.left, p, q)
-        if root.val < p.val and root.val < q.val:
-            return self.lowestCommonAncestor(root.right, p, q)
-        return root
+        if cur.val < p.val and cur.val < q.val:           # 右
+            right = self.traversal(cur.right, p, q)
+            if right is not None:
+                return right
+
+        return cur
+
+    def lowestCommonAncestor(self, root, p, q):
+        return self.traversal(root, p, q)
 ```
 
-迭代法：
+迭代法（版本二）精简
 ```python
 class Solution:
-    """二叉搜索树的最近公共祖先 迭代法"""
+    def lowestCommonAncestor(self, root, p, q):
+        if root.val > p.val and root.val > q.val:
+            return self.lowestCommonAncestor(root.left, p, q)
+        elif root.val < p.val and root.val < q.val:
+            return self.lowestCommonAncestor(root.right, p, q)
+        else:
+            return root
 
-    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
-        while True:
+```
+
+迭代法
+```python
+class Solution:
+    def lowestCommonAncestor(self, root, p, q):
+        while root:
             if root.val > p.val and root.val > q.val:
                 root = root.left
             elif root.val < p.val and root.val < q.val:
                 root = root.right
             else:
                 return root
-```
+        return None
 
+
+```
 ## Go 
 
 递归法：
diff --git a/problems/0236.二叉树的最近公共祖先.md b/problems/0236.二叉树的最近公共祖先.md
index 33201def..0ebd5566 100644
--- a/problems/0236.二叉树的最近公共祖先.md
+++ b/problems/0236.二叉树的最近公共祖先.md
@@ -274,25 +274,44 @@ class Solution {
 ```
 
 ## Python 
-
+递归法（版本一）
 ```python
 class Solution:
-    """二叉树的最近公共祖先 递归法"""
-
-    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
-        if not root or root == p or root == q:
+    def lowestCommonAncestor(self, root, p, q):
+        if root == q or root == p or root is None:
             return root
-        
+
         left = self.lowestCommonAncestor(root.left, p, q)
         right = self.lowestCommonAncestor(root.right, p, q)
-        
-        if left and right:
-            return root
-        if left:
-            return left
-        return right
-```
 
+        if left is not None and right is not None:
+            return root
+
+        if left is None and right is not None:
+            return right
+        elif left is not None and right is None:
+            return left
+        else: 
+            return None
+```
+递归法（版本二）精简
+```python
+class Solution:
+    def lowestCommonAncestor(self, root, p, q):
+        if root == q or root == p or root is None:
+            return root
+
+        left = self.lowestCommonAncestor(root.left, p, q)
+        right = self.lowestCommonAncestor(root.right, p, q)
+
+        if left is not None and right is not None:
+            return root
+
+        if left is None:
+            return right
+        return left
+
+```
 ## Go 
 
 ```Go
diff --git a/problems/0450.删除二叉搜索树中的节点.md b/problems/0450.删除二叉搜索树中的节点.md
index 66d5b3cf..3d73598d 100644
--- a/problems/0450.删除二叉搜索树中的节点.md
+++ b/problems/0450.删除二叉搜索树中的节点.md
@@ -324,88 +324,90 @@ class Solution {
 ```
 
 ## Python 
-
+递归法（版本一）
 ```python
 class Solution:
-    def deleteNode(self, root: Optional[TreeNode], key: int) -> Optional[TreeNode]:
-        if not root : return  None # 节点为空，返回
-        if root.val < key :
-            root.right = self.deleteNode(root.right, key)
-        elif root.val > key :
+    def deleteNode(self, root, key):
+        if root is None:
+            return root
+        if root.val == key:
+            if root.left is None and root.right is None:
+                return None
+            elif root.left is None:
+                return root.right
+            elif root.right is None:
+                return root.left
+            else:
+                cur = root.right
+                while cur.left is not None:
+                    cur = cur.left
+                cur.left = root.left
+                return root.right
+        if root.val > key:
             root.left = self.deleteNode(root.left, key)
-        else:
-            # 当前节点的左子树为空，返回当前的右子树
-            if not root.left : return root.right
-            # 当前节点的右子树为空，返回当前的左子树
-            if not root.right: return root.left
-            # 左右子树都不为空，找到右孩子的最左节点 记为p
-            node = root.right
-            while node.left :
-                node = node.left
-            # 将当前节点的左子树挂在p的左孩子上
-            node.left = root.left
-            # 当前节点的右子树替换掉当前节点，完成当前节点的删除
-            root = root.right
+        if root.val < key:
+            root.right = self.deleteNode(root.right, key)
         return root
 ```
 
-**普通二叉树的删除方式**
+递归法（版本二）
 ```python
 class Solution:
-    def deleteNode(self, root: Optional[TreeNode], key: int) -> Optional[TreeNode]:
-        if not root: return root
-        if root.val == key:
-            if not root.right:  # 这里第二次操作目标值：最终删除的作用
+    def deleteNode(self, root, key):
+        if root is None:  # 如果根节点为空，直接返回
+            return root
+        if root.val == key:  # 找到要删除的节点
+            if root.right is None:  # 如果右子树为空，直接返回左子树作为新的根节点
                 return root.left
-            tmp = root.right
-            while tmp.left:
-                tmp = tmp.left
-            root.val, tmp.val = tmp.val, root.val  # 这里第一次操作目标值：交换目标值其右子树最左面节点。
-            
-        root.left = self.deleteNode(root.left, key)
-        root.right = self.deleteNode(root.right, key)
+            cur = root.right
+            while cur.left:  # 找到右子树中的最左节点
+                cur = cur.left
+            root.val, cur.val = cur.val, root.val  # 将要删除的节点值与最左节点值交换
+        root.left = self.deleteNode(root.left, key)  # 在左子树中递归删除目标节点
+        root.right = self.deleteNode(root.right, key)  # 在右子树中递归删除目标节点
         return root
+
 ```
 
 **迭代法**
 ```python
 class Solution:
-    def deleteNode(self, root: Optional[TreeNode], key: int) -> Optional[TreeNode]:
-        # 找到节点后分两步，1. 把节点的左子树和右子树连起来，2. 把右子树跟父节点连起来
-        # root is None
-        if not root: return root
-        p = root
-        last = None
-        while p:
-            if p.val==key:
-                # 1. connect left to right
-                # right is not None -> left is None | left is not None
-                if p.right:
-                    if p.left:
-                        node = p.right
-                        while node.left:
-                            node = node.left
-                        node.left = p.left
-                    right = p.right
-                else:
-                # right is None -> right=left
-                    right = p.left
-                # 2. connect right to last
-                if last==None:
-                    root = right
-                elif last.val>key:
-                    last.left = right
-                else:
-                    last.right = right
-                # 3. return
+    def deleteOneNode(self, target: TreeNode) -> TreeNode:
+        """
+        将目标节点（删除节点）的左子树放到目标节点的右子树的最左面节点的左孩子位置上
+        并返回目标节点右孩子为新的根节点
+        是动画里模拟的过程
+        """
+        if target is None:
+            return target
+        if target.right is None:
+            return target.left
+        cur = target.right
+        while cur.left:
+            cur = cur.left
+        cur.left = target.left
+        return target.right
+
+    def deleteNode(self, root: TreeNode, key: int) -> TreeNode:
+        if root is None:
+            return root
+        cur = root
+        pre = None  # 记录cur的父节点，用来删除cur
+        while cur:
+            if cur.val == key:
                 break
+            pre = cur
+            if cur.val > key:
+                cur = cur.left
             else:
-                # Update last and continue
-                last = p
-                if p.val>key:
-                    p = p.left
-                else:
-                    p = p.right
+                cur = cur.right
+        if pre is None:  # 如果搜索树只有头结点
+            return self.deleteOneNode(cur)
+        # pre 要知道是删左孩子还是右孩子
+        if pre.left and pre.left.val == key:
+            pre.left = self.deleteOneNode(cur)
+        if pre.right and pre.right.val == key:
+            pre.right = self.deleteOneNode(cur)
         return root
 ```
 
diff --git a/problems/0501.二叉搜索树中的众数.md b/problems/0501.二叉搜索树中的众数.md
index e467bf8a..1da32343 100644
--- a/problems/0501.二叉搜索树中的众数.md
+++ b/problems/0501.二叉搜索树中的众数.md
@@ -528,8 +528,7 @@ class Solution {
 
 ## Python 
 
-> 递归法
-> 常量空间，递归产生的栈不算
+递归法（版本一）利用字典
 
 ```python
 # Definition for a binary tree node.
@@ -538,77 +537,108 @@ class Solution {
 #         self.val = val
 #         self.left = left
 #         self.right = right
+from collections import defaultdict
+
 class Solution:
-    def __init__(self):
-        self.pre = TreeNode()
-        self.count = 0
-        self.max_count = 0
-        self.result = []
+    def searchBST(self, cur, freq_map):
+        if cur is None:
+            return
+        freq_map[cur.val] += 1  # 统计元素频率
+        self.searchBST(cur.left, freq_map)
+        self.searchBST(cur.right, freq_map)
 
-    def findMode(self, root: TreeNode) -> List[int]:
-        if not root: return None
-        self.search_BST(root)
-        return self.result
-        
-    def search_BST(self, cur: TreeNode) -> None:
-        if not cur: return None
-        self.search_BST(cur.left)
-        # 第一个节点
-        if not self.pre:
-            self.count = 1
-        # 与前一个节点数值相同
-        elif self.pre.val == cur.val:
-            self.count += 1 
-        # 与前一个节点数值不相同
-        else:
-            self.count = 1
-        self.pre = cur
+    def findMode(self, root):
+        freq_map = defaultdict(int)  # key:元素，value:出现频率
+        result = []
+        if root is None:
+            return result
+        self.searchBST(root, freq_map)
+        max_freq = max(freq_map.values())
+        for key, freq in freq_map.items():
+            if freq == max_freq:
+                result.append(key)
+        return result
 
-        if self.count == self.max_count:
-            self.result.append(cur.val)
-        
-        if self.count > self.max_count:
-            self.max_count = self.count
-            self.result = [cur.val]	# 清空self.result，确保result之前的的元素都失效
-        
-        self.search_BST(cur.right)
 ```
 
+递归法（版本二）利用二叉搜索树性质
 
-> 迭代法-中序遍历
-> 利用二叉搜索树特性，在历遍过程中更新结果，一次历遍
-> 但需要使用额外空间存储历遍的节点
 ```python
 class Solution:
-    def findMode(self, root: TreeNode) -> List[int]:
-        stack = []
+    def __init__(self):
+        self.maxCount = 0  # 最大频率
+        self.count = 0  # 统计频率
+        self.pre = None
+        self.result = []
+
+    def searchBST(self, cur):
+        if cur is None:
+            return
+
+        self.searchBST(cur.left)  # 左
+        # 中
+        if self.pre is None:  # 第一个节点
+            self.count = 1
+        elif self.pre.val == cur.val:  # 与前一个节点数值相同
+            self.count += 1
+        else:  # 与前一个节点数值不同
+            self.count = 1
+        self.pre = cur  # 更新上一个节点
+
+        if self.count == self.maxCount:  # 如果与最大值频率相同，放进result中
+            self.result.append(cur.val)
+
+        if self.count > self.maxCount:  # 如果计数大于最大值频率
+            self.maxCount = self.count  # 更新最大频率
+            self.result = [cur.val]  # 很关键的一步，不要忘记清空result，之前result里的元素都失效了
+
+        self.searchBST(cur.right)  # 右
+        return
+
+    def findMode(self, root):
+        self.count = 0
+        self.maxCount = 0
+        self.pre = None  # 记录前一个节点
+        self.result = []
+
+        self.searchBST(root)
+        return self.result
+```
+迭代法
+```python
+class Solution:
+    def findMode(self, root):
+        st = []
         cur = root
         pre = None
-        maxCount, count = 0, 0
-        res = []
-        while cur or stack:
-            if cur:  # 指针来访问节点，访问到最底层
-                stack.append(cur)
-                cur = cur.left
-            else:  # 逐一处理节点
-                cur = stack.pop()
-                if pre == None:  # 第一个节点
+        maxCount = 0  # 最大频率
+        count = 0  # 统计频率
+        result = []
+
+        while cur is not None or st:
+            if cur is not None:  # 指针来访问节点，访问到最底层
+                st.append(cur)  # 将访问的节点放进栈
+                cur = cur.left  # 左
+            else:
+                cur = st.pop()
+                if pre is None:  # 第一个节点
                     count = 1
                 elif pre.val == cur.val:  # 与前一个节点数值相同
                     count += 1
-                else:
+                else:  # 与前一个节点数值不同
                     count = 1
-                if count == maxCount:
-                    res.append(cur.val)
-                if count > maxCount:
-                    maxCount = count
-                    res.clear()
-                    res.append(cur.val)
+
+                if count == maxCount:  # 如果和最大值相同，放进result中
+                    result.append(cur.val)
+
+                if count > maxCount:  # 如果计数大于最大值频率
+                    maxCount = count  # 更新最大频率
+                    result = [cur.val]  # 很关键的一步，不要忘记清空result，之前result里的元素都失效了
 
                 pre = cur
-                cur = cur.right
-        return res	
-	
+                cur = cur.right  # 右
+
+        return result
 ```
 ## Go 
 
diff --git a/problems/0530.二叉搜索树的最小绝对差.md b/problems/0530.二叉搜索树的最小绝对差.md
index cd24c6fa..3e4391d6 100644
--- a/problems/0530.二叉搜索树的最小绝对差.md
+++ b/problems/0530.二叉搜索树的最小绝对差.md
@@ -237,66 +237,82 @@ class Solution {
 ```
 ## Python 
 
-递归
+递归法（版本一）利用中序递增，结合数组
 ```python
 class Solution:
-    def getMinimumDifference(self, root: TreeNode) -> int:
-        res = []   
-        r = float("inf")
-        def buildaList(root):  //把二叉搜索树转换成有序数组
-            if not root: return None
-            if root.left: buildaList(root.left)  //左
-            res.append(root.val)  //中
-            if root.right: buildaList(root.right)  //右
-            return res
-            
-        buildaList(root)
-        for i in range(len(res)-1):  // 统计有序数组的最小差值
-            r = min(abs(res[i]-res[i+1]),r)
-        return r
-        
-        
-class Solution: # 双指针法，不用数组 (同Carl写法) - 更快
-    def getMinimumDifference(self, root: Optional[TreeNode]) -> int:
-        global pre,minval
-        pre = None
-        minval = 10**5
-        self.traversal(root)
-        return minval
+    def __init__(self):
+        self.vec = []
 
-    def traversal(self,root):
-        global pre,minval
-        if not root: return None
+    def traversal(self, root):
+        if root is None:
+            return
         self.traversal(root.left)
-        if pre and root.val-pre.val<minval: 
-            minval = root.val-pre.val
-        pre = root
-        self.traversal(root.right)            
+        self.vec.append(root.val)  # 将二叉搜索树转换为有序数组
+        self.traversal(root.right)
+
+    def getMinimumDifference(self, root):
+        self.vec = []
+        self.traversal(root)
+        if len(self.vec) < 2:
+            return 0
+        result = float('inf')
+        for i in range(1, len(self.vec)):
+            # 统计有序数组的最小差值
+            result = min(result, self.vec[i] - self.vec[i - 1])
+        return result
+
 ```
 
 
-迭代法-中序遍历
+
+递归法（版本二）利用中序递增，找到该树最小值
 ```python 
 class Solution:
-    def getMinimumDifference(self, root: TreeNode) -> int:
+    def __init__(self):
+        self.result = float('inf')
+        self.pre = None
+
+    def traversal(self, cur):
+        if cur is None:
+            return
+        self.traversal(cur.left)  # 左
+        if self.pre is not None:  # 中
+            self.result = min(self.result, cur.val - self.pre.val)
+        self.pre = cur  # 记录前一个
+        self.traversal(cur.right)  # 右
+
+    def getMinimumDifference(self, root):
+        self.traversal(root)
+        return self.result
+
+        
+```
+
+迭代法
+```python 
+class Solution:
+    def getMinimumDifference(self, root):
         stack = []
         cur = root
         pre = None
         result = float('inf')
-        while cur or stack:
-            if cur: # 指针来访问节点，访问到最底层
-                stack.append(cur)
-                cur = cur.left
-            else: # 逐一处理节点
+
+        while cur is not None or len(stack) > 0:
+            if cur is not None:
+                stack.append(cur)  # 将访问的节点放进栈
+                cur = cur.left  # 左
+            else:
                 cur = stack.pop()
-                if pre: # 当前节点和前节点的值的差值
-                    result = min(result, abs(cur.val - pre.val))
+                if pre is not None:  # 中
+                    result = min(result, cur.val - pre.val)
                 pre = cur
-                cur = cur.right
+                cur = cur.right  # 右
+
         return result
+
+
         
 ```
-
 ## Go： 
 
 中序遍历，然后计算最小差值
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 ad5310e1..ad4decc5 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
@@ -200,8 +200,30 @@ class Solution {
 ```
 
 ## Python 
-**递归**
-
+递归法(版本一)
+```python
+# 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:
+        self.pre = 0  # 记录前一个节点的数值
+        self.traversal(root)
+        return root
+    def traversal(self, cur):
+        if cur is None:
+            return        
+        self.traversal(cur.right)
+        cur.val += self.pre
+        self.pre = cur.val
+        self.traversal(cur.left)
+    
+        
+```
+递归法（版本二）
 ```python
 # Definition for a binary tree node.
 # class TreeNode:
@@ -234,7 +256,32 @@ class Solution:
         return root 
         
 ```
-**迭代**
+迭代法（版本一）
+```python
+class Solution:
+    def __init__(self):
+        self.pre = 0  # 记录前一个节点的数值
+    
+    def traversal(self, root):
+        stack = []
+        cur = root
+        while cur or stack:
+            if cur:
+                stack.append(cur)
+                cur = cur.right  # 右
+            else:
+                cur = stack.pop()  # 中
+                cur.val += self.pre
+                self.pre = cur.val
+                cur = cur.left  # 左
+    
+    def convertBST(self, root):
+        self.pre = 0
+        self.traversal(root)
+        return root
+
+```            
+迭代法（版本二）
 ```python
 class Solution:
     def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
diff --git a/problems/0669.修剪二叉搜索树.md b/problems/0669.修剪二叉搜索树.md
index 18d8a0cc..95372d61 100644
--- a/problems/0669.修剪二叉搜索树.md
+++ b/problems/0669.修剪二叉搜索树.md
@@ -271,64 +271,72 @@ class Solution {
 
 ## Python
 
-**递归**
-
+递归法（版本一）
 ```python
-# 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:
-        '''
-        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 is None:
+            return None
         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
+            # 寻找符合区间 [low, high] 的节点
             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
+        if root.val > high:
+            # 寻找符合区间 [low, high] 的节点
             return self.trimBST(root.left, low, high)
+        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
 
-        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
 ```
-
-**迭代**
-
+递归法（版本二）精简
 ```python
 class Solution:
-    def trimBST(self, root: Optional[TreeNode], low: int, high: int) -> Optional[TreeNode]:
-        if not root: return root
-        # 处理头结点，让root移动到[L, R] 范围内，注意是左闭右开
-        while root and (root.val < low or root.val > high):
-            if root.val < low:  # 小于L往右走
-                root = root.right
-            else:  # 大于R往左走
-                root = root.left
-        # 此时root已经在[L, R] 范围内，处理左孩子元素小于L的情况
+    def trimBST(self, root: TreeNode, low: int, high: int) -> TreeNode:
+        if root is None:
+            return None
+        if root.val < low:
+            return self.trimBST(root.right, low, high)
+        if root.val > high:
+            return self.trimBST(root.left, low, high)
+        root.left = self.trimBST(root.left, low, high)
+        root.right = self.trimBST(root.right, low, high)
+        return root
+
+
+```
+
+迭代法
+```python
+class Solution:
+    def trimBST(self, root: TreeNode, L: int, R: int) -> TreeNode:
+        if not root:
+            return None
+        
+        # 处理头结点，让root移动到[L, R] 范围内，注意是左闭右闭
+        while root and (root.val < L or root.val > R):
+            if root.val < L:
+                root = root.right  # 小于L往右走
+            else:
+                root = root.left  # 大于R往左走
+        
         cur = root
+        
+        # 此时root已经在[L, R] 范围内，处理左孩子元素小于L的情况
         while cur:
-            while cur.left and cur.left.val < low:
+            while cur.left and cur.left.val < L:
                 cur.left = cur.left.right
             cur = cur.left
-        # 此时root已经在[L, R] 范围内，处理右孩子大于R的情况
+        
         cur = root
+        
+        # 此时root已经在[L, R] 范围内，处理右孩子大于R的情况
         while cur:
-            while cur.right and cur.right.val > high:
+            while cur.right and cur.right.val > R:
                 cur.right = cur.right.left
             cur = cur.right
+        
         return root
+
 ```
 
 ## Go
diff --git a/problems/0701.二叉搜索树中的插入操作.md b/problems/0701.二叉搜索树中的插入操作.md
index 4e834201..1ba7461f 100644
--- a/problems/0701.二叉搜索树中的插入操作.md
+++ b/problems/0701.二叉搜索树中的插入操作.md
@@ -256,132 +256,103 @@ class Solution {
 -----
 ## Python 
 
-**递归法** - 有返回值
-
+递归法（版本一）
 ```python
-# 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 insertIntoBST(self, root: TreeNode, val: int) -> TreeNode:
-        # 返回更新后的以当前root为根节点的新树，方便用于更新上一层的父子节点关系链
+    def __init__(self):
+        self.parent = None
 
-        # Base Case
-        if not root: return TreeNode(val)
+    def traversal(self, cur, val):
+        if cur is None:
+            node = TreeNode(val)
+            if val > self.parent.val:
+                self.parent.right = node
+            else:
+                self.parent.left = node
+            return
 
-        # 单层递归逻辑:
-        if val < root.val: 
-            # 将val插入至当前root的左子树中合适的位置
-            # 并更新当前root的左子树为包含目标val的新左子树
-            root.left = self.insertIntoBST(root.left, val)
+        self.parent = cur
+        if cur.val > val:
+            self.traversal(cur.left, val)
+        if cur.val < val:
+            self.traversal(cur.right, val)
 
-        if root.val < val:
-            # 将val插入至当前root的右子树中合适的位置
-            # 并更新当前root的右子树为包含目标val的新右子树
-            root.right = self.insertIntoBST(root.right, val)
-
-        # 返回更新后的以当前root为根节点的新树
+    def insertIntoBST(self, root, val):
+        self.parent = TreeNode(0)
+        if root is None:
+            return TreeNode(val)
+        self.traversal(root, val)
         return root
+
 ```
 
-**递归法** - 无返回值
+递归法（版本二）
 ```python
 class Solution:
-    def insertIntoBST(self, root: TreeNode, val: int) -> TreeNode:
-        if not root: 
+    def insertIntoBST(self, root, val):
+        if root is None:
             return TreeNode(val)
         parent = None
-        def __traverse(cur: TreeNode, val: int) -> None: 
-            # 在函数运行的同时把新节点插入到该被插入的地方. 
-            nonlocal parent
-            if not cur: 
-                new_node = TreeNode(val)
-                if parent.val < val: 
-                    parent.right = new_node
-                else: 
-                    parent.left = new_node
-                return 
-
-            parent = cur # 重点: parent的作用只有运行到上面if not cur:才会发挥出来.
-            if cur.val < val: 
-                __traverse(cur.right, val)
-            else: 
-                __traverse(cur.left, val)
-            return
-        __traverse(root, val)
-        return root
-```
-
-**递归法** - 无返回值 - another easier way
-```python
-class Solution:
-    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
-        newNode = TreeNode(val)
-        if not root: return newNode
-        
-        if not root.left and val < root.val:
-            root.left = newNode
-        if not root.right and val > root.val:
-            root.right = newNode
-        
-        if val < root.val:
-            self.insertIntoBST(root.left, val)
-        if val > root.val:
-            self.insertIntoBST(root.right, val)
-        
-        return root
-```
-
-**递归法** - 无返回值 有注释 不用Helper function
-```python
-class Solution:
-    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
-        if not root:    # for root==None
-            return TreeNode(val)
-        if root.val<val:
-            if root.right==None:    # find the parent
-                root.right = TreeNode(val)
-            else:   # not found, keep searching
-                self.insertIntoBST(root.right, val)
-        if root.val>val:
-            if root.left==None: # found the parent
-                root.left = TreeNode(val)
-            else:   # not found, keep searching
-                self.insertIntoBST(root.left, val)
-        # return the final tree
-        return root
-```
-
-**迭代法**  
-与无返回值的递归函数的思路大体一致 
-```python
-class Solution:
-    def insertIntoBST(self, root: TreeNode, val: int) -> TreeNode:
-        if not root: 
-            return TreeNode(val)
-        parent = None # 此步可以省略
         cur = root
-
-        # 用while循环不断地找新节点的parent
-        while cur: 
-            parent = cur  # 首先保存当前非空节点作为下一次迭代的父节点
-            if cur.val < val: 
-                cur = cur.right
-            elif cur.val > val: 
+        while cur:
+            parent = cur
+            if val < cur.val:
                 cur = cur.left
-
-        # 运行到这意味着已经跳出上面的while循环, 
-        # 同时意味着新节点的parent已经被找到.
-        # parent已被找到, 新节点已经ready. 把两个节点黏在一起就好了. 
-        if parent.val > val: 
+            else:
+                cur = cur.right
+        if val < parent.val:
             parent.left = TreeNode(val)
-        else: 
+        else:
             parent.right = TreeNode(val)
-        
         return root
+```
+
+递归法（版本三）
+```python
+class Solution:
+    def insertIntoBST(self, root: Optional[TreeNode], val: int) -> Optional[TreeNode]:
+        if root is None or root.val == val:
+            return TreeNode(val)
+        elif root.val > val:
+            if root.left is None:
+                root.left = TreeNode(val)
+            else:
+                self.insertIntoBST(root.left, val)
+        elif root.val < val:
+            if root.right is None:
+                root.right = TreeNode(val)
+            else:
+                self.insertIntoBST(root.right, val)
+        return root
+```
+
+
+
+迭代法
+```python
+class Solution:
+    def insertIntoBST(self, root, val):
+        if root is None:  # 如果根节点为空，创建新节点作为根节点并返回
+            node = TreeNode(val)
+            return node
+
+        cur = root
+        parent = root  # 记录上一个节点，用于连接新节点
+        while cur is not None:
+            parent = cur
+            if cur.val > val:
+                cur = cur.left
+            else:
+                cur = cur.right
+
+        node = TreeNode(val)
+        if val < parent.val:
+            parent.left = node  # 将新节点连接到父节点的左子树
+        else:
+            parent.right = node  # 将新节点连接到父节点的右子树
+
+        return root
+
             
 ```
 -----