Exercise1_3_33

package chapterone;

/***
*@author LuodiJack
*
*ResizingArrayDeque()         创建空的双向队列
*boolean isEmpty()                 双向队列是否为空
*int size()                                双向队列的元素数量
*void pushLeft(Item item)                  向左端添加一个新元素
*void pushRight(Item item)                 向右端添加一个新元素
*Item popLeft()                    从左端删除一个元素
*Item popRight()                   从右端删除一个元素
*
*  测试不完全，如有bug,敬请赐教!
*/

import java.util.Iterator;

public class ResizingArrayDeque<Item> implements Iterable<Item> {
private int left;
private int right;
private Item[] a;// entries
private int N;// size

@SuppressWarnings("unchecked")
public ResizingArrayDeque() {
a = (Item[]) new Object[2];
left = 0;
right = 1;
N = 0;
}

public boolean isEmpty() {
return N == 0;
}

public int size() {
return N;
}

private void resize(int len) {
@SuppressWarnings("unchecked")
Item[] tmp = (Item[]) new Object[len];
int i=left, j=tmp.length / 2;
for ( ; i < right; i++, j++) {
tmp[j] = a[i];
}
left=tmp.length / 2;
right=j;
a = tmp;
}

public void pushLeft(Item item) {
if ((left - 1) < 0) {
resize(2 * a.length);
}
a[left--] = item;
N++;
}

public void pushRight(Item item) {
if ((right + 1) >= a.length) {
resize(2 * a.length);
}
a[right++] = item;
N++;
}

public Item popLeft() {
if (isEmpty()) {
throw new RuntimeException("The Deque is Empty!");
}
Item item = a[++left];
a[left] = null;// 防止对象游离
if ((right - left) <= a.length / 4) {
resize(a.length / 2);
}
N--;
if (N == 0) {
left = right - 1;
}
return item;
}

public Item popRight() {
if (isEmpty()) {
throw new RuntimeException("The Deque is Empty!");
}
Item item = a[--right];
a[right] = null;// 防止对象游离
if ((right - left) <= a.length / 4) {
resize(a.length / 2);
}
N--;
if (N == 0) {
right = left + 1;
}
return item;
}

public Iterator<Item> iterator() {
return new ResizingArrayDequenIterator();
}

private class ResizingArrayDequenIterator implements Iterator<Item> {
private int i = left;

public boolean hasNext() {
return (i + 1) < right;
}

public Item next() {
return a[++i];
}

}

}