算法－第四版－练习1.3.33解答

问题

Deque。一个双向队列(或者称为deque)和栈或队列类似，但它同时支持在两端添加或删除元素。Deque能够存储一组元素并支持如下API。

/** * -----------------------------------------------------  * public class Deque<Item> implements Iterable<Item> * -----------------------------------------------------  *              Deque()             创建空双向队列 *      boolean isEmpty()            双向队列是否为空 *          int size()               双向队列中元素的数量 *         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()            从左羰删除一个元素 *         Item popRight()           从右羰删除一个元素 * -----------------------------------------------------  */

编写一个使用双向链表实现这份API的Deque类。以及一个使用动态数据组调整实现这份API的ResizingArrayDeque类。

解决思路

该问题解决起来并不难。使用双向链表时需要注意每个结点的双向引用，不要只添加单向的引用或者只删除单向的引用。使用动态数组时注意，何时调整数组的大小以及将数组大小调整为多大。本实现添加元素时，只有在头部或者尾部没有空间时，才进行调整，调整为队列中元素的个数的3倍，前后各留队列中元素个数的长度。在删除元素，当元素的数量小于总长度的1/6时，进行调整，将长度调整为元素个数的3倍，同样前后各留元素个数的长度。注意最小不小于3。

代码

一共三个文件，分别为Deque类，ResizingArrayDeque类和测试文件E10333类。

Deque类。

package com.furzoom.lab.algs.ch103;import java.util.Iterator;/** * -----------------------------------------------------  * public class Deque<Item> implements Iterable<Item> * -----------------------------------------------------  *              Deque()              创建空双向队列 *      boolean isEmpty()            双向队列是否为空 *          int size()               双向队列中元素的数量 *         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()            从左羰删除一个元素 *         Item popRight()           从右羰删除一个元素 * -----------------------------------------------------  */public class Deque<Item> implements Iterable<Item> {private Node head = null;private Node tail = null;private int size = 0;private class Node {public Item item;public Node prev;public Node next;}public boolean isEmpty() {return head == null;}public int size() {return size;}public void pushLeft(Item item) {Node node = new Node();node.item = item;node.prev = null;if (isEmpty()) {head = tail = node;node.next = null;} else {head.prev = node;node.next = head;head = node;}size++;}public void pushRight(Item item) {Node node = new Node();node.item = item;node.next = null;if (isEmpty()) {head = tail = node;node.prev = null;} else {tail.next = node;node.prev = tail;tail = node;}size++;}public Item popLeft() {if (isEmpty()) {return null;} else {Item e = head.item;if (size() == 1) {head = tail = null;} else {head = head.next;head.prev.next = null;head.prev = null;}size--;return e;}}public Item popRight() {if (isEmpty()) {return null;} else {Item e = tail.item;if (size() == 1) {head = tail = null;} else {tail = tail.prev;tail.next.prev = null;tail.next = null;}size--;return e;}}@Overridepublic Iterator<Item> iterator() {return new Iter();}private class Iter implements Iterator<Item> {private Node current = head;@Overridepublic boolean hasNext() {return current != null;}@Overridepublic Item next() {Item e = current.item;current = current.next;return e;}}}

ResizingArrayDeque类。

package com.furzoom.lab.algs.ch103;import java.util.Iterator;public class ResizingArrayDeque<Item> implements Iterable<Item> {private int head;private int tail;private Item[] deque;@SuppressWarnings("unchecked")public ResizingArrayDeque() {deque = (Item[])new Object[3];head = 1;tail = 1;}public boolean isEmpty() {return head == tail;}public int size() {return tail - head;}public void pushLeft(Item item) {if (head == 0) {resize(3 * size());}deque[--head] = item;}public void pushRight(Item item) {if (tail == deque.length) {resize(3 * size());}deque[tail++] = item;}public Item popLeft() {if (isEmpty()) {return null;}if (size() * 6 < deque.length) {resize(size() * 3);}return deque[head++];}public Item popRight() {if (isEmpty()) {return null;}if (size() * 6 < deque.length) {resize(size() * 3);}return deque[--tail];}@SuppressWarnings("unchecked")private void resize(int size) {if (size < 3) {size = 3;}Item tmp[] = (Item[])new Object[size];int j = size / 3;for (int i = head; i < tail; i++) {tmp[j++] = deque[i];}deque = tmp;head = size / 3;tail = j;}@Overridepublic Iterator<Item> iterator() {return new Iter();}private class Iter implements Iterator<Item> {private int current = head;@Overridepublic boolean hasNext() {return current < tail;}@Overridepublic Item next() {Item e = deque[current++];return e;}}}

测试代码：

package com.furzoom.lab.algs.ch103;import java.util.Iterator;public class E10333 {public static void main(String[] args) {//Deque<String> deque = new Deque<String>();ResizingArrayDeque<String> deque = new ResizingArrayDeque<String>();deque.pushLeft("c");deque.pushLeft("b");deque.pushRight("d");deque.pushRight("e");System.out.println("deque size: " + deque.size());Iterator<String> it = deque.iterator();while (it.hasNext()) {System.out.println(it.next());}System.out.println("Pop up from right: ");while (!deque.isEmpty()) {System.out.println(deque.popRight());}deque.pushLeft("c");deque.pushLeft("b");deque.pushRight("d");deque.pushRight("e");System.out.println("Pop up from left: ");while (!deque.isEmpty()) {System.out.println(deque.popLeft());}}}

结果

deque size: 4bcdePop up from right: edcbPop up from left: bcde

算法－第四版－1.3 背包、队列和栈－习题索引汇总

算法－第四版习题索引汇总