隊列(Quene)的特徵就是“先進先出”,隊列把所有操作限制在"只能線上性結構的兩端"進行,更具體一點:添加元素必須線上性表尾部進行,而刪除元素只能線上性表頭部進行。
先抽象介面IQuene<T>
namespace 棧與隊列{ public interface IQuene<T> { /// <summary> /// 取得隊列實際元素的個數 /// </summary> /// <returns></returns> public int Count(); /// <summary> /// 判斷隊列是否為空白 /// </summary> /// <returns></returns> public bool IsEmpty(); /// <summary> /// 清空隊列 /// </summary> public void Clear(); /// <summary> /// 入隊(即向隊列尾部添加一個元素) /// </summary> /// <param name="item"></param> public void Enquene(T item); /// <summary> /// 出隊(即從隊列頭部刪除一個元素) /// </summary> /// <returns></returns> public T Dequene(); /// <summary> /// 取得隊列頭部第一元素 /// </summary> /// <returns></returns> public T Peek(); }}
下面是基於數組實現的:
實現思路:用一個數組存放所有元素,同時設定二個關鍵變數front與rear用於記錄隊列“頭”與“尾”的元素下標,當有元素入列時rear加1,當有元素出隊時front+1,而rear-front即為隊列實際元素的總數.
但有一種“隊列偽滿”的特殊情況要注意,如:
這張圖上面的部分:假設經過入隊、出隊一番折騰後,rear已經指向數組的下標最大值,而front指向在中間(即front之間的元素已經出隊不用考慮了,相當於front下標前面的記憶體地區空閑),如果這時再有一個元素入列,rear+1就超出數組下標的最大值了,但是從圖上一眼就能看出,實際上front前面還空著一堆位置可以重複利用,隊列並非真正的“滿”--這種情況稱為偽滿,為瞭解決這個問題,我們可以把數組想象為首尾相接的迴圈結構,即圖中下面部分,這時候可以讓rear重新指向到0,以便重複利用閒置位置。
所以:入列時rear++的操作,應該稍做修正,當rear到數組下標最大值時,讓它置0,以便能迴圈利用 (見後面的代碼)
另外還有一個問題:最開始時front與rear都為-1,即front==rear時表示隊列為空白,改成迴圈以後,有可能會出現rear在迴圈過程中碰到front的情況,即真正意義的上"滿"狀態,這時rear也同樣等於front,這樣就無法單純的用rear==front來判斷是滿,還是空?這時可以浪費一個元素的位置,認為當rear+1==front時,隊列就已經滿了,雖然犧牲了一個元素的空間,但卻換來了邏輯的正確性,還是值得的。
完整實現如下:
using System;using System.Text;namespace 棧與隊列{ /// <summary> /// 迴圈順序隊列 /// </summary> /// <typeparam name="T"></typeparam> public class CSeqQueue<T>:IQueue<T> { private int maxsize; private T[] data; private int front; private int rear; public CSeqQueue(int size) { data = new T[size]; maxsize = size; front = rear = -1; } public int Count() { if (rear > front) { return rear - front; } else { return (rear - front + maxsize) % maxsize; } } public void Clear() { front = rear = -1; } public bool IsEmpty() { return front == rear; } public bool IsFull() { if (front != -1) //如果已經有元素出隊過 { return (rear + 1) % maxsize == front;//為了區分與IsEmpty的區別,有元素出隊過以後,就只有浪費一個位置來保持邏輯正確性. } else { return rear == maxsize - 1; } } public void Enqueue(T item) { if (IsFull()) { Console.WriteLine("Queue is full"); return; } if (rear == maxsize - 1) //如果rear到頭了,則迴圈重來(即解決偽滿問題) { rear = 0; } else { rear++; } data[rear] = item; } public T Dequeue() { if (IsEmpty()) { Console.WriteLine("Queue is empty"); return default(T); } if (front == maxsize - 1) //如果front到頭了,則重新置0 { front = 0; } else { front++; } return data[front]; } public T Peek() { if (IsEmpty()) { Console.WriteLine("Queue is empty!"); return default(T); } return data[(front + 1) % maxsize]; } public override string ToString() { if (IsEmpty()) { return "queue is empty."; } StringBuilder sb = new StringBuilder(); if (rear > front) { for (int i = front + 1; i <= rear; i++) { sb.Append(this.data[i].ToString() + ","); } } else { for (int i = front + 1; i < maxsize; i++) { sb.Append(this.data[i].ToString() + ","); } for (int i = 0; i <= rear; i++) { sb.Append(this.data[i].ToString() + ","); } } return "front = " + this.front + " \t rear = " + this.rear + "\t count = " + this.Count() + "\t data = " + sb.ToString().Trim(','); } }}
測試程式碼片段:
CSeqQueue<int> queue = new CSeqQueue<int>(5); queue.Enqueue(1); queue.Enqueue(2); queue.Enqueue(3); queue.Enqueue(4); Console.WriteLine(queue);//front = -1 rear = 3 count = 4 data = 1,2,3,4 queue.Dequeue(); Console.WriteLine(queue);//front = 0 rear = 3 count = 3 data = 2,3,4 queue.Dequeue(); Console.WriteLine(queue);//front = 1 rear = 3 count = 2 data = 3,4 queue.Enqueue(5); Console.WriteLine(queue);//front = 1 rear = 4 count = 3 data = 3,4,5 queue.Enqueue(6); Console.WriteLine(queue);//front = 1 rear = 0 count = 4 data = 3,4,5,6 queue.Enqueue(7); //Queue is full Console.WriteLine(queue);//front = 1 rear = 0 count = 4 data = 3,4,5,6 queue.Dequeue(); queue.Enqueue(7); Console.WriteLine(queue);//front = 2 rear = 1 count = 4 data = 4,5,6,7 queue.Clear(); Console.WriteLine(queue);//queue is empty. queue.Enqueue(1); queue.Enqueue(2); queue.Enqueue(3); queue.Enqueue(4); Console.WriteLine(queue);//front = -1 rear = 3 count = 4 data = 1,2,3,4 queue.Enqueue(5); Console.WriteLine(queue);//front = -1 rear = 4 count = 5 data = 1,2,3,4,5 queue.Enqueue(6); //Queue is full Console.WriteLine(queue);//front = -1 rear = 4 count = 5 data = 1,2,3,4,5 queue.Dequeue(); queue.Dequeue(); queue.Dequeue(); queue.Dequeue(); Console.WriteLine(queue);//front = 3 rear = 4 count = 1 data = 5 queue.Dequeue(); Console.WriteLine(queue);//queue is empty. queue.Enqueue(0); queue.Enqueue(1); queue.Enqueue(2); queue.Enqueue(3); queue.Enqueue(4); //Queue is full Console.WriteLine(queue);//front = 4 rear = 3 count = 4 data = 0,1,2,3 Console.WriteLine(queue.Peek());//0 queue.Dequeue(); Console.WriteLine(queue);//front = 0 rear = 3 count = 3 data = 1,2,3 queue.Dequeue(); Console.WriteLine(queue);//front = 1 rear = 3 count = 2 data = 2,3 queue.Dequeue(); Console.WriteLine(queue);//front = 2 rear = 3 count = 1 data = 3 queue.Dequeue(); Console.WriteLine(queue);//queue is empty. queue.Enqueue(9); Console.WriteLine(queue);//front = 3 rear = 4 count = 1 data = 9 Console.ReadLine();
當然,隊列也可以用鏈表來實現,相對要容易很多。
先定義鏈表中的節點Node.cs
namespace 棧與隊列{ public class Node<T> { private T data; private Node<T> next; public Node(T data, Node<T> next) { this.data = data; this.next = next; } public Node(Node<T> next) { this.next = next; this.data = default(T); } public Node(T data) { this.data = data; this.next = null; } public Node() { this.data = default(T); this.next = null; } public T Data { get { return this.data; } set { this.data = value; } } public Node<T> Next { get { return next; } set { next = value; } } }}
為了方便,定義了很多建構函式的重載版本,當然這些只是浮雲,重點是理解結構:data用來儲存資料,next指出下一個節點是誰
鏈式隊列的完整實現LinkQueue.cs
using System;using System.Text;namespace 棧與隊列{ public class LinkQueue:IQueue { private Node front;//隊列頭 private Node rear;//隊列尾 private int num;//隊列元素個數 /// /// 構造器 /// public LinkQueue() { //初始時front,rear置為null,num置0 front = rear = null; num = 0; } public int Count() { return this.num; } public void Clear() { front = rear = null; num = 0; } public bool IsEmpty() { return (front == rear && num == 0); } //入隊 public void Enqueue(T item) { Node q = new Node(item); if (rear == null)//第一個元素入列時 { front = rear = q; } else { //把新元素掛到鏈尾 rear.Next = q; //修正rear指向為最後一個元素 rear = q; } //元素總數+1 num++; } //出隊 public T Dequeue() { if (IsEmpty()) { Console.WriteLine("Queue is empty!"); return default(T); } //取鏈首元素 Node p = front; //鏈頭指向後移一位 front = front.Next; //如果此時鏈表為空白,則同步修正rear if (front == null) { rear = null; } num--;//個數-1 return p.Data; } public T Peek() { if (IsEmpty()) { Console.WriteLine("Queue is empty!"); return default(T); } return front.Data; } public override string ToString() { if (IsEmpty()) { Console.WriteLine("Queue is empty!"); } StringBuilder sb = new StringBuilder(); Node node = front; sb.Append(node.Data.ToString()); while (node.Next!=null) { sb.Append("," + node.Next.Data.ToString()); node = node.Next; } return sb.ToString().Trim(','); } }}