Java数据结构与算法(4)

队列： 先进先出(FIFO)。

优先级队列： 在优先级队列中，数据项按照关键字的值有序，关键字最小的数据项总在对头，数据项插入的时候会按照顺序插入到合适的位置以确保队列的顺序，从后往前将小于插入项的数据项后移。在图的最小生成树算法中应用优先级队列。

示例代码：

package chap04.Queue;

class Queue {
    private int maxSize;
    private long[] queArray;
    private int front;
    private int rear;
    private int nItems;

    public Queue(int s) {
        maxSize = s;
        queArray = new long[maxSize];
        front = 0;
        rear = -1;
        nItems = 0;
    }

    // 插入方法，队尾是数组的最后一项
    public void insert(long j) {
        if (rear == maxSize - 1) {
            rear = -1;
        }
        queArray[++rear] = j;
        nItems++;
    }

    // 先进先出
    public long remove() {
        long temp = queArray[front++];
        if (front == maxSize) {
            front = 0;
        }
        nItems--;
        return temp;
    }

    public long peekFront() {
        return queArray[front];
    }

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

    public boolean isFull() {
        return (nItems == maxSize);
    }

    public int size() {
        return nItems;
    }
}

class PriorityQ {
    private int maxSize;
    private long[] queArray;
    private int nItems;

    public PriorityQ(int s) {
        maxSize = s;
        queArray = new long[maxSize];
        nItems = 0;
    }

    // 插入方法，从大到小排列
    public void insert(long item) {
        int j;

        if (nItems == 0) {
            queArray[nItems++] = item;  
        }
        else {
            for (j = nItems - 1; j >= 0; j--) {
                if (item > queArray[j]) { // if new item larger,
                    queArray[j + 1] = queArray[j]; 
                }
                else {
                    break; 
                }
            } 
            queArray[j + 1] = item; 
            nItems++;
        } 
    } 

    // 按照优先级从后往前移除，不再跟先进还是后进有关
    public long remove() {
        return queArray[--nItems];
    }

    public long peekMin() {
        return queArray[nItems - 1];
    }

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

    public boolean isFull() {
        return (nItems == maxSize);
    }
}

class QueueApp {
    public static void main(String[] args) {
        Queue theQueue = new Queue(5);

        theQueue.insert(10);
        theQueue.insert(20);
        theQueue.insert(30);
        theQueue.insert(40);

        theQueue.remove();
        theQueue.remove();
        theQueue.remove();

        theQueue.insert(50);
        theQueue.insert(60);
        theQueue.insert(70);
        theQueue.insert(80);

        while (!theQueue.isEmpty()) {
            long n = theQueue.remove();
            System.out.print(n); // 40, 50, 60, 70, 80
            System.out.print(" ");
        }
        System.out.println("");
        
        PriorityQ thePQ = new PriorityQ(5);
        thePQ.insert(30);
        thePQ.insert(50);
        thePQ.insert(10);
        thePQ.insert(40);
        thePQ.insert(20);

        while (!thePQ.isEmpty()) {
            long item = thePQ.remove();
            System.out.print(item + " "); // 10, 20, 30, 40, 50
        }  
        System.out.println("");
    }
}