Update 二叉树的统一迭代法.md 微调 stack 赋值语句

problems/二叉树的统一迭代法.md

@@ -316,7 +316,7 @@ class Solution:
 class Solution:
     def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
         values = []
-        stack = [] if root is None else [root]
+        stack = [root] if root else []
         popped_nodes = set() # 用于记录一个节点是否被 pop() 过
 
         while stack:
@@ -345,7 +345,7 @@ class Solution:
 class Solution:
     def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
         values = []
-        stack = [] if root is None else [root]
+        stack = [root] if root else []
         popped_nodes = set() # 用于记录一个节点是否被 pop() 过
 
         while stack: