diff --git a/notes/算法.md b/notes/算法.md
index 2ddd1f1b..27fb7023 100644
--- a/notes/算法.md
+++ b/notes/算法.md
@@ -301,10 +301,12 @@ public class Queue<Item> {
     }
 
     // 出队列
-    public Item dequeue(){
+    public Item dequeue() {
+        if (isEmpty()) return null;
         Node node = first;
         first = first.next;
         N--;
+        if (isEmpty()) last = null;
         return node.item;
     }
 }