diff --git a/notes/剑指 offer 题解.md b/notes/剑指 offer 题解.md
index 2dc15d12..6d27553b 100644
--- a/notes/剑指 offer 题解.md	
+++ b/notes/剑指 offer 题解.md	
@@ -393,11 +393,14 @@ public void push(int node) {
     in.push(node);
 }
 
-public int pop() {
+public int pop() throws Exception{
     if (out.isEmpty()) {
         while (!in.isEmpty()) {
             out.push(in.pop());
         }
+        if (out.isEmpty()){
+            throw new Exception("queue is empty");
+        }
     }
     return out.pop();
 }