下压(LIFO)栈（能够动态调整数组大小的实现）

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//下压(LIFO)栈（能够动态调整数组大小的实现）/* * 每项操作的用时都与集合大小无关 * 空间需求总是不超过集合大小乘以一个常数 */package Algorithms;import java.util.Iterator;import java.util.NoSuchElementException;import edu.princeton.cs.algs4.StdIn;import edu.princeton.cs.algs4.StdOut;public class P88_ResizingArrayStack<Item> implements Iterable<Item>{private Item[] a;private int n;public P88_ResizingArrayStack(){a = (Item[]) new Object[2];n = 0;}public boolean isEmpty(){return n == 0;}public int size(){return n;}private void resize(int capacity){assert capacity >= n;//将栈移动到一个大小capacity的新数组Item[] temp = (Item[]) new Object[capacity];for (int i = 0; i < n; i++){temp[i] = a[i];}a = temp;}//将元素添加到栈顶public void push(Item item){if (n == a.length){resize(2 * a.length);a[n++] = item;}}//从栈顶删除元素public Item pop(){if (isEmpty()){throw new NoSuchElementException("Stack underflow");}Item item = a[n - 1];a[n - 1] = null;   //避免对象游离n--;if(n > 0 && n == a.length / 4){resize(a.length / 2);}return item;}public Item peek(){if (isEmpty()){throw new NoSuchElementException("Stack underflow");}return a[n - 1];}public Iterator<Item> iterator(){return new ReverseArrayIterator();}//支持后进先出的迭代private class ReverseArrayIterator implements Iterator<Item>{private int i;public ReverseArrayIterator(){i = n - 1;}public boolean hasNext() {            return i >= 0;        }        public void remove() {            throw new UnsupportedOperationException();        }        public Item next() {            if (!hasNext()) throw new NoSuchElementException();            return a[i--];        }}public static void main(String[] args) {// TODO Auto-generated method stubP88_ResizingArrayStack<String> stack = new P88_ResizingArrayStack<String>();while (!StdIn.isEmpty()){String item = StdIn.readString();if (!item.equals("-")){stack.push(item);}else if (!stack.isEmpty()){StdOut.print(stack.pop() + " ");}}StdOut.println("(" + stack.size() + " left on stack)");}}
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