From ca576c5bd0a62769c4b080028e7092d7fb945890 Mon Sep 17 00:00:00 2001
From: Eyjan_Huang <81480748+Eyjan-Huang@users.noreply.github.com>
Date: Fri, 20 Aug 2021 03:29:26 +0800
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添加限制条件，仅在栈不为空的情况下才允许后续pop()和peek()的操作
---
 problems/0232.用栈实现队列.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/problems/0232.用栈实现队列.md b/problems/0232.用栈实现队列.md
index 46d884d3..27a3de4e 100644
--- a/problems/0232.用栈实现队列.md
+++ b/problems/0232.用栈实现队列.md
@@ -304,6 +304,9 @@ class MyQueue:
         2. 如果out没有元素，就把in里面的元素（除了第一个）依次pop后装进out里面
         3. 直接把in剩下的元素pop出来，就是queue头部的
         """
+        if self.empty:
+            return None
+        
         if self.stack_out:
             return self.stack_out.pop()
         else:
@@ -317,6 +320,9 @@ class MyQueue:
         1. 查out有没有元素，有就把最上面的返回
         2. 如果out没有元素，就把in最下面的返回
         """
+        if self.empty:
+            return None
+        
         if self.stack_out:
             return self.stack_out[-1]
         else: