Update stack and queue.

docs/chapter_stack_and_queue/deque.md

@@ -38,35 +38,34 @@ comments: true
     ```java title="deque.java"
     /* 初始化双向队列 */
     Deque<Integer> deque = new LinkedList<>();
-    
+
     /* 元素入队 */
     deque.offerLast(2);
     deque.offerLast(5);
     deque.offerLast(4);
     deque.offerFirst(3);
     deque.offerFirst(1);
-    System.out.println("队列 deque = " + deque);
-    
+    System.out.println("双向队列 deque = " + deque);
+
     /* 访问队首元素 */
     int peekFirst = deque.peekFirst();
     System.out.println("队首元素 peekFirst = " + peekFirst);
     int peekLast = deque.peekLast();
     System.out.println("队尾元素 peekLast = " + peekLast);
-    
+
     /* 元素出队 */
     int pollFirst = deque.pollFirst();
-    System.out.println("队首出队元素 pollFirst = " + pollFirst + 
-                     "，队首出队后 deque = " + deque);
+    System.out.println("队首出队元素 pollFirst = " + pollFirst + "，队首出队后 deque = " + deque);
     int pollLast = deque.pollLast();
-    System.out.println("队尾出队元素 pollLast = " + pollLast + 
-                     "，队尾出队后 deque = " + deque);
-    
-    /* 获取队列的长度 */
+    System.out.println("队尾出队元素 pollLast = " + pollLast + "，队尾出队后 deque = " + deque);
+
+    /* 获取双向队列的长度 */
     int size = deque.size();
-    System.out.println("队列长度 size = " + size);
-    
-    /* 判断队列是否为空 */
+    System.out.println("双向队列长度 size = " + size);
+
+    /* 判断双向队列是否为空 */
     boolean isEmpty = deque.isEmpty();
+    System.out.println("双向队列是否为空 = " + isEmpty);
     ```
 
 === "C++"