Update 0669.修剪二叉搜索树.md 添加迭代法python3代码

problems/0669.修剪二叉搜索树.md

@@ -299,6 +299,32 @@ class Solution:
             return root
 ```
 
+**迭代**
+```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
+        cur = root
+        while cur:
+            while cur.left and cur.left.val < low:
+                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
+        while cur:
+            while cur.right and cur.right.val > high:
+                cur.right = cur.right.left
+            cur = cur.right
+        return root
+```
+
 ## Go 
 
 ```go