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# 双向队列
对于队列，我们只能在头部删除或在尾部添加元素，而「双向队列 Deque」更加灵活，
< p align = "center" > Fig. 双向队列的操作 </ p >
## 双向队列常用操作
双向队列的常用操作见下表，方法名需根据编程语言设定来具体确定。
< p align = "center" > Table. 双向队列的常用操作 </ p >
< div class = "center-table" markdown >
| 方法 | 描述 |
| ------------ | ---------------- |
| offerFirst() | 将元素添加至队首 |
| offerLast() | 将元素添加至队尾 |
| pollFirst() | 删除队首元素 |
| pollLast() | 删除队尾元素 |
| peekFirst() | 访问队首元素 |
| peekLast() | 访问队尾元素 |
| size() | 获取队列的长度 |
| isEmpty() | 判断队列是否为空 |
</ div >
相同地，我们可以直接使用编程语言实现好的双向队列类。
=== "Java"
```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);
/* 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);
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 */
int size = deque.size();
System.out.println("队列长度 size = " + size);
/* 判断队列是否为空 */
boolean isEmpty = deque.isEmpty();
```
=== "C++"
```cpp title="deque.cpp"
```
=== "Python"
```python title="deque.py"
```
