package practice;

 import java.util.Iterator;

 //栈

 public class MyStack<T> implements Iterable<T> {

     private T[] a;

     private int p = 0;

     @SuppressWarnings("unchecked")

     public MyStack() {

         a = (T[]) new Object[1];

     }

     public void push(T elements) {

         if (a.length == p) {resize(2*a.length);}

         a[p++] = elements;

     }

     public T pop() {

         if (a.length/4 == p) {resize(a.length/2);}

         return a[--p];

     }

     @SuppressWarnings("unchecked")

     public void resize(int length) {

         T[] b = (T[]) new Object[length]; //将数组长度改变

         for (int i = 0; i < p; i++) { //将数组复制

             b[i] = a[i];

         }

         a = b; //让a指向b的内存空间

     }

     public boolean isEmpty() {

         return p == 0;

     }

     public int size() {

         return p;

     }

     @Override

     public Iterator<T> iterator() { //使数据结构可以让foreach访问

         return new ArrayIterator();

     }

     private class ArrayIterator implements Iterator<T>{

         private int p2 = 0;

         @Override

         public boolean hasNext() { //return false 则foreach终止

             return p2 != p;

         }

         @Override

         public T next() { //foreach 寻找下一个元素

             return a[p2++];

         }

     }

 }