本章内容主要来自算法(第四版) Robert Sedgewick著 1.3小节

栈(后进先出)的基本功能

public class Stack<Item> implements Iterable<Item>{
   Stack()
   void push(Item)
   Item pop()
   boolean isEmpty()
   int size()
}

队列(先进先出):

public class Queue<Item> implements Iterable<Item>{
    Queue()
    void enqueue(Item item)
    Item deque()
    boolean isEmpty()
    int size()
}

关于Iterable和Iterator:

interface　　　　　　method
java.lang.Iterable => iterator()
java.util.Iterator => hasNext()、next()、remove()

foreach实质

  Stack<String> collection = new Stack<String>();
  ...
  for(String s : collection){
      System.out.println(s);
  }
  ...
  等价于如下代码=======================================
  Iterator<String> i = collection.iterator();
  while(i.hasNext()){
      System.out.println(i.next());
  }

所以实现Iterable需实现Iterator()方法，返回的是实现java.util.Iterator接口的内部类对象，注意内部类可以访问外部类实例域
数组没有实现该接口也能使用foreach，留待后答

数组版栈实现

import java.util.Iterator;

public class Test {
    public static void main(String[] args) {

    }
}
//未重写toString()等方法
class Stack<T> implements Iterable<T>{
    private int N =0;
    private T[] t;
    public Stack() {
        t = (T[])new Object[8];   //此处将默认设为8
    }
    public Stack(int length) {
        t = (T[])new Object[length];   //此处将默认设为8
    }
    private void resize(int length) {
        //将栈移动到一个长度为length的新数组
        T[] temp = (T[])new Object[length];
        for(int i=0;i<N;i++) {
            temp[i] = t[i];
        }
        t = temp;
    }
    public void push(T element) {
        if(N==t.length) {
            resize(2*t.length+2);
        }
        t[N++] = element;
    }
    public T pop() {
        T a = null;
        if(N>0){
            a=t[--N];
            t[N]=null;
            if(N>8&&N<t.length/4){
                resize(t.length/2);
            }
        }
        return a;
            
        
    }
    public boolean isEmpty() {
        return N==0;
    }
    public int size() {
        return N;
    }
    @Override
    public Iterator<T> iterator() {
        return new ReverseArrayIterator();
    }
    //JDK 1.8不用覆盖remove()方法了
    private class ReverseArrayIterator implements Iterator<T>{
        private int i = N;
        @Override
        public boolean hasNext() {
            return i > 0;
        }
        @Override
        public T next() {
            return t[--i];
        }
    }
}

链表版栈实现

class Stack<T> implements Iterable<T>{
    private Node first;
    private int N;
    private class Node{
        T element;
        Node next;
    }
    public boolean isEmpty(){
        return N==0;
    }
    public int size(){
        return N;
    }
    public void push(T element){
        Node oldfirst = first;
        first = new Node();
        first.element = element;
        first.next = oldfirst;
        N++;
    }
    public T pop(){
        //未显示考虑栈为空的状态
        T element = first.element;
        first = first.next;
        N--;
        return element;
    }
    @Override
    public Iterator<T> iterator() {
        return new ListIterator();
    }
    private class ListIterator implements Iterator<T>{
        private Node current = first;
        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public T next() {
            T element = current.element;
            current = current.next;
            return element;
        }
        
    }
}

数组版队列实现

import java.util.Iterator;

public class Test {
    public static void main(String[] args) {

    }
}
//循环列表
class Queue<T> implements Iterable<T>{
    private int head;
    private int tail;
    private int N;
    T[] t;
    public Queue(){
        N = 0;
        head = 0;
        tail = 0;
        t = (T[])new Object[8];
    }
    public Queue(int length){
        N = 0;
        head = 0;
        tail = 0;
        t = (T[])new Object[length];
    }
    public void enqueue(T element){
        if(N<t.length){
            if(N==0){
                t[head]=element;
            }
            else{
                tail = (++tail) % t.length;
                t[tail] = element;
            }
            N++;
        }
        else{
            //队列已满
        }
    }
    public void dequeue(){
        if(N>0){
            //N=1说明首尾指向同一位
            if(N==1){
                t[head]=null;
                N--;
            }
            else{
                //为null防止对象游离
                t[head]=null;
                head = (++head)%t.length;
                N--;
            }
        }
        else{
            //队列为空
        }
    }
    public int size(){
        return N;
        
    }
    
    @Override
    public Iterator<T> iterator() {
        return new FirstinfirstoutArrayIterator();
    }
    private class FirstinfirstoutArrayIterator implements Iterator<T>{
        private int i = 0;
        @Override
        public boolean hasNext() {
            return i < N;
        }
        @Override
        public T next() {
            T element = t[(i+head)%t.length];    //从头元素开始获取元素.
            i++;
            return element;
        }
    }
}

链表版队列实现

import java.util.Iterator;

public class Test {
    public static void main(String[] args) {

    }
}
class Queue<T> implements Iterable<T>{
    private Node first;
    private Node last;
    private int N;
    private class Node{
        T element;
        Node next;
    }
    public boolean isEmpty(){
        return first == null;
    }
    public int size(){
        return N;
    }
    public void enqueue(T element){
        Node oldlast = last;
        last = new Node();
        last.element = element;
        last.next = null;
        if(isEmpty()) first = last;
        else oldlast.next = last;
        N++;
    }
    public T dequeue(){
        if(isEmpty()){
            first = null;
            last = null;
            return null;
        }
        T element = first.element;
        first = first.next;
        N--;
        if(N==0){
            first = null;
            last = null;
        }
        return element;
    }
    @Override
    public Iterator<T> iterator() {
        return new ListIterator();
    }
    private class ListIterator implements Iterator<T>{
        private Node current = first;
        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public T next() {
            T element = current.element;
            current = current.next;
            return element;
        }
        
    }
}

java中不允许泛型数组，需要类型转换例如 a=(Type[])new Object[N]