3 --- stack and queue

自己写的，两个基本类型的，基于链表的实现。

public class Stack implements Cloneable {
 Node top; // pointing to the toppest node on a stack

 Node pop() {
  if (top != null) {
   Node item = top;
   top = top.next;
   return item;
  }
  return null;
 }

 void push(int k) {
  Node t = new Node(k);
  t.next = top;
  top = t;
 }

 Object peek() {
  return top.data;
 }

 public Stack clone() throws CloneNotSupportedException {
  return (Stack) super.clone();
 }
 
 boolean isEmpty(){
  return (this.top ==null);
 }
}

public class Queue {
 // in this implementation, tail.next is always null;, so that we can enqueue() at tail
 // thus, delete at head();
 Node head, tail;
 void enqueue(Node item){// enqueue at tail. 
  if(tail ==null){
   tail = item;
   head = item;
  }else{
   tail.next = item;
   item.next = null;
   tail = item;
  }
 }
 Object dequeue(){//  dequeue at head 
  // head point to the just added item. or the first in a single list
  if(head!=null){
   Object item = head;
   head = head.next;
   return item;
  }
  return null;
 }
}

Learned:
1: Constructor call must be the first statement in a constructor