diff --git "a/notes/\347\256\227\346\263\225.md" "b/notes/\347\256\227\346\263\225.md"
index 2ddd1f1..27fb702 100644
--- "a/notes/\347\256\227\346\263\225.md"
+++ "b/notes/\347\256\227\346\263\225.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;
     }
 }