diff --git a/problems/0101.对称二叉树.md b/problems/0101.对称二叉树.md
index 40249bb7..fd2d1987 100644
--- a/problems/0101.对称二叉树.md
+++ b/problems/0101.对称二叉树.md
@@ -437,6 +437,31 @@ class Solution:
         return True
 ```
 
+层次遍历
+```python
+class Solution:
+    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
+        if not root:
+            return True
+
+        que = [root]
+        while que:
+            this_level_length = len(que)
+            for i in range(this_level_length // 2):
+                # 要么其中一个是None但另外一个不是
+                if (not que[i] and que[this_level_length - 1 - i]) or (que[i] and not que[this_level_length - 1 - i]):
+                    return False
+                # 要么两个都不是None
+                if que[i] and que[i].val != que[this_level_length - 1 - i].val:
+                    return False
+            for i in range(this_level_length):
+                if not que[i]: continue
+                que.append(que[i].left)
+                que.append(que[i].right)
+            que = que[this_level_length:]
+        return True
+```
+
 ## Go
 
 ```go
diff --git a/problems/0110.平衡二叉树.md b/problems/0110.平衡二叉树.md
index 3aa815ab..5bdd4f1b 100644
--- a/problems/0110.平衡二叉树.md
+++ b/problems/0110.平衡二叉树.md
@@ -531,40 +531,26 @@ class Solution:
 迭代法：
 ```python
 class Solution:
-    def isBalanced(self, root: TreeNode) -> bool:
-        st = []
+    def isBalanced(self, root: Optional[TreeNode]) -> bool:
         if not root:
             return True
-        st.append(root)
-        while st:
-            node = st.pop() #中
-            if abs(self.getDepth(node.left) - self.getDepth(node.right)) > 1:
-                return False
-            if node.right:
-                st.append(node.right) #右（空节点不入栈）
-            if node.left:
-                st.append(node.left) #左（空节点不入栈）
-        return True
-    
-    def getDepth(self, cur):
-        st = []
-        if cur:
-            st.append(cur)
-        depth = 0
-        result = 0
-        while st:
-            node = st.pop()
+
+        height_map = {}
+        stack = [root]
+        while stack:
+            node = stack.pop()
             if node:
-                st.append(node) #中
-                st.append(None)
-                depth += 1
-                if node.right: st.append(node.right) #右
-                if node.left: st.append(node.left) #左
+                stack.append(node)
+                stack.append(None)
+                if node.left: stack.append(node.left)
+                if node.right: stack.append(node.right)
             else:
-                node = st.pop()
-                depth -= 1
-            result = max(result, depth)
-        return result
+                real_node = stack.pop()
+                left, right = height_map.get(real_node.left, 0), height_map.get(real_node.right, 0)
+                if abs(left - right) > 1:
+                    return False
+                height_map[real_node] = 1 + max(left, right)
+        return True
 ```