1 public class UseLinkedListImplementStack {
2 private ListNode head;
3 private int length ;
4 public UseLinkedListImplementStack() {
5 this.head = null ;
6 this.length = 0;
7 }
8 /* H
9 (last in) 1->2->3->4 (1st in)
10 o/H
11 */
12 public void push(int val){
13 ListNode node = new ListNode(val) ;
14 node.next = head ;
15 head = node ;
16 length++;
17 }
18
19 public Integer pop(){
20 if (head == null) return null;
21 int value = head.val ;
22 head = head.next;
23 length-- ;
24 return value ;
25 }
26
27 public Integer peek(){
28 if (head == null) return null ;
29 return head.val ;
30 }
31
32 public boolean isEmpty(){
33 return this.length <= 0 ;
34 }
35
36 public int getSize(){
37 return this.length ;
38 }
39
40 public static void main(String[] args) {
41 UseLinkedListImplementStack stack = new UseLinkedListImplementStack();
42 stack.push(1);
43 stack.push(3);
44 System.out.println(stack.getSize());
45 System.out.println(stack.peek());
46 System.out.println(stack.pop());
47 stack.push(4);
48 System.out.println(stack.peek());
49 }
50 }
