8.1：双向链表实现双端队列

　　双端队列，玩的head 和tail指针

底层都是双链表调整

　　1、双向链表

节点
1 public static class Node<T> {
2         public T value;
3         public Node<T> last;
4         public Node<T> next;
5 
6         public Node(T data) {
7             value = data;
8         }
9     }

 双向链表实现节点定义

 1     public static class DoubleEndsQueue<T> {
 2         public Node<T> head;
 3         public Node<T> tail;
 4 
 5         public void addFromHead(T value) {
 6             Node<T> cur = new Node<T>(value);
 7             if (head == null) {
 8                 head = cur;
 9                 tail = cur;
10             } else {
11                 cur.next = head;
12                 head.last = cur;
13                 head = cur;
14             }
15         }
16 
17         public void addFromBottom(T value) {
18             Node<T> cur = new Node<T>(value);
19             if (head == null) {
20                 head = cur;
21                 tail = cur;
22             } else {
23                 cur.last = tail;
24                 tail.next = cur;
25                 tail = cur;
26             }
27         }
28 
29         public T popFromHead() {
30             if (head == null) {
31                 return null;
32             }
33             Node<T> cur = head;
34             if (head == tail) {
35                 head = null;
36                 tail = null;
37             } else {
38                 head = head.next;
39                 cur.next = null;
40                 head.last = null;
41             }
42             return cur.value;
43         }
44 
45         public T popFromBottom() {
46             if (head == null) {
47                 return null;
48             }
49             Node<T> cur = tail;
50             if (head == tail) {
51                 head = null;
52                 tail = null;
53             } else {
54                 tail = tail.last;
55                 tail.next = null;
56                 cur.last = null;
57             }
58             return cur.value;
59         }
60 
61         public boolean isEmpty() {
62             return head == null;
63         }
64 
65     }

定义双端队列
 1 public static class MyQueue<T> {
 2         private DoubleEndsQueue<T> queue;
 3 
 4         public MyQueue() {
 5             queue = new DoubleEndsQueue<T>();
 6         }
 7 
 8         public void push(T value) {
 9             queue.addFromHead(value);
10         }
11 
12         public T poll() {
13             return queue.popFromBottom();
14         }
15 
16         public boolean isEmpty() {
17             return queue.isEmpty();
18         }
19 
20     }