数据结构与算法——队列-编程知识

数据结构与算法——队列

概述

　　计算机科学中，queue 是以顺序的方式维护的一组数据集合，在一端添加数据，从另一端移除数据。添加的一端称为尾，移除的一端称为头。

功能

　　插入offer(value : E) : boolean
　　取值并移除poll() : E
　　取值peek() : E
　　判断是否为空isEmpty() : boolean
　　判断队列是否满isfull() : boolean

接口代码

public interface Queue<E> {/*** 向队列尾插入值* @param value 待插入值* @return 插入成功返回 true, 插入失败返回 false*/boolean offer(E value);/*** 从对列头获取值, 并移除* @return 如果队列非空返回对头值, 否则返回 null*/E poll();/*** 从对列头获取值, 不移除* @return 如果队列非空返回对头值, 否则返回 null*/E peek();/*** 检查队列是否为空* @return 空返回 true, 否则返回 false*/boolean isEmpty();/*** 检查队列是否已满* @return 满返回 true, 否则返回 false*/boolean isFull();
}

链表实现

　　利用单向环形带哨兵链表实现

代码

import java.util.Iterator;
import java.util.StringJoiner;/*** 基于单向环形链表实现* @param <E> 队列中元素类型*/
public class LinkedListQueue<E>implements Queue<E>, Iterable<E> {private static class Node<E> {E value;Node<E> next;public Node(E value, Node<E> next) {this.value = value;this.next = next;}}private final Node<E> head = new Node<>(null, null);private Node<E> tail = head;int size = 0;private int capacity = Integer.MAX_VALUE;{tail.next = head;}public LinkedListQueue() {}public LinkedListQueue(int capacity) {this.capacity = capacity;}@Overridepublic boolean offer(E value) {if (isFull()) {return false;}Node<E> added = new Node<>(value, head);tail.next = added;tail = added;size++;return true;}@Overridepublic E poll() {if (isEmpty()) {return null;}Node<E> first = head.next;head.next = first.next;if (first == tail) {tail = head;}size--;return first.value;}@Overridepublic E peek() {if (isEmpty()) {return null;}return head.next.value;}@Overridepublic boolean isEmpty() {return head == tail;}@Overridepublic boolean isFull() {return size == capacity;}@Overridepublic Iterator<E> iterator() {return new Iterator<E>() {Node<E> p = head.next;@Overridepublic boolean hasNext() {return p != head;}@Overridepublic E next() {E value = p.value;p = p.next;return value;}};}@Overridepublic String toString() {StringJoiner sj = new StringJoiner(",");for (E e : this) {sj.add(e.toString());}return sj.toString();}
}

数组实现

　　利用移位与相与操作，解决超长队列问题。

代码

import java.util.Iterator;/*** 仅用 head, tail 判断空满, head, tail 需要换算成索引值** @param <E> 队列中元素类型*/
public class ArrayQueue3<E> implements Queue<E>, Iterable<E> {/*求模运算：- 如果除数是 2 的 n 次方- 那么被除数的后 n 位即为余数 (模)- 求被除数的后 n 位方法： 与 2^n-1 按位与*/private final E[] array;private int head = 0;private int tail = 0;@SuppressWarnings("all")public ArrayQueue3(int c) {// 1. 抛异常/*if ((capacity & capacity - 1) != 0) {throw new IllegalArgumentException("capacity 必须是2的幂");}*/// 2. 改成 2^n    13 -> 16   22 -> 32int n = (int) (Math.log10(c-1) / Math.log10(2)) + 1;array = (E[]) new Object[1 << n];/*2^4     = 162^4.?   = 302^5     = 32(int)log2(30) + 12log2(x) = log10(x) / log10(2)110      2^1100     2^21000    2^3*/}/*head = 0tail = 3  % 3 = 0capacity=30   1   2d   b   c*/@Overridepublic boolean offer(E value) {if (isFull()) {return false;}array[tail & (array.length - 1)] = value;tail++;return true;}@Overridepublic E poll() {if (isEmpty()) {return null;}int idx = head & (array.length - 1);E value = array[idx];array[idx] = null; // help GChead++;return value;}@Overridepublic E peek() {if (isEmpty()) {return null;}return array[head & (array.length - 1)];}@Overridepublic boolean isEmpty() {return head == tail;}@Overridepublic boolean isFull() {return tail - head == array.length;}@Overridepublic Iterator<E> iterator() {return new Iterator<E>() {int p = head;@Overridepublic boolean hasNext() {return p != tail;}@Overridepublic E next() {E value = array[p & (array.length - 1)];p++;return value;}};}  
}