java实现栈和队列

 1 class Node {
 2     int val;
 3     Node next;
 4     
 5     Node(int x) {
 6         val = x;
 7         next = null;
 8     }
 9 }
10 
11 class Stack {
12     Node top;
13     
14     public Node peek() {
15         if (top != null) {
16             return top;
17         }
18         return null;
19     }
20     
21     public Node pop() {
22         if (top == null) {
23             return null;
24         } else {
25             Node temp = new Node(top.val);
26             top = top.next;
27             return temp;
28         }
29     }
30     public void push(Node n) {
31         if (n != null) {
32             n.next = top;
33             top = n;
34         }
35     }
36 }
37 
38 class Queue {
39     Node first, last;
40     
41     public void enqueue(Node n) {
42         if (first == null) {
43             first = n;
44             last = first;
45         } else {
46             last.next = n;
47             last = n;
48         }
49     }
50     
51     public Node dequeue() {
52         if (first == null) {
53             return null;
54         } else {
55             Node temp = new Node(first.val);
56             first = first.next;
57             return temp;
58         }
59     } 
60 }