day18 循环队列

1. 循环队列特点

• 用图去理解代码，循环队列我觉得一定要结合图去理解是容易的。入队时判断队是否已满再入队(顺序：先入值再修改指针)；出队时判断队是否为空再出队(顺序：先出值再修改指针)

• 队空 对满的判定条件 （1）队满:(rear + 1) % MAX_SIZE = front;
  （2）队空：front = rear 这里的队满判断是牺牲了一个空间，还有其他方式也是可以判断队满的。

2. 取模

我们可以从链队列可以看出，出队头指针加1，入队尾指针加1，头/尾指针不做任何处理，则会一直无限制向上加，会导致空间浪费。通过取模运行，就可以避免上述的问题，循环队列最大长度假设只有MAX_SIZE=10,取模则可以使头/尾指针做循环，进而空间最大化的使用。

3.代码

package datastructure.queue;
public class CircleIntQueue {
    /**
     * The total space. One space can never be used.
     */
    public static final int TOTAL_SPACE = 10;

    int[] data;

    /**
     * The index for calculating the head. The actual head is head % TOTAL_SPACE.
     */
    int head;

    /**
     * The index for calculating the tail.
     */
    int tail;

    public CircleIntQueue(){
        data = new int[TOTAL_SPACE];
        head = 0;
        tail = 0;
    }

    /**
     * enqueue
     * @param paraValue The value of the new node.
     */
    public void enqueue(int paraValue){
        if ((tail+1)%TOTAL_SPACE == head){
            System.out.println("Queue full.");
            return;
        }

        data[tail%TOTAL_SPACE] = paraValue;
        tail++;
    }

    public int dequeue(){
        if (head == tail){
            System.out.println("No element in the queue");
            return -1;
        }
        int resultValue = data[head%TOTAL_SPACE];
        head++;

        return resultValue;
    }

    @Override
    public String toString(){
        String resultString = "";

        if (head == tail){
            return "empty";
        }

        for (int i = head; i < tail; i++){
            resultString += data[i%TOTAL_SPACE] + ", ";
        }
        return resultString;
    }

    public static void main(String[] args) {
        CircleIntQueue tempQueue = new CircleIntQueue();
        System.out.println("Initialized, the list is: " + tempQueue.toString());

        for (int i = 0; i < 5; i++) {
            tempQueue.enqueue(i + 1);
        }
        System.out.println("Enqueue, the queue is: " + tempQueue.toString());

        int tempValue = tempQueue.dequeue();
        System.out.println("Dequeue " + tempValue + ", the queue is: " + tempQueue.toString());

        for (int i = 0; i < 6; i++) {
            tempQueue.enqueue(i + 10);
            System.out.println("Enqueue, the queue is: " + tempQueue.toString());
        }

        for (int i = 0; i < 3; i++) {
            tempValue = tempQueue.dequeue();
            System.out.println("Dequeue " + tempValue + ", the queue is: " + tempQueue.toString());
        }

        for (int i = 0; i < 6; i++) {
            tempQueue.enqueue(i + 100);
            System.out.println("Enqueue, the queue is: " + tempQueue.toString());
        }
    }
}

字符类型：

package datastructure.queue;
public class CircleCharQueue {
    public static final int TOTAL_SPACE = 10;

    char[] data;
    int head;
    int tail;

    /**
     * The constructor
     */
    public CircleCharQueue() {
        data = new char[TOTAL_SPACE];
        head = 0;
        tail = 0;
    }

    /**
     * Enqueue.
     * @param paraValue The value of the new node.
     */
    public void enqueue(char paraValue) {
        if ((tail + 1) % TOTAL_SPACE == head) {
            System.out.println("Queue full.");
            return;
        }

        data[tail % TOTAL_SPACE] = paraValue;
        tail++;
    }

    /**
     * Dequeue
     * @return The value at the head.
     */
    public char dequeue() {
        if (head == tail) {
            System.out.println("No element in the queue");
            return '\0';
        }

        char resultValue = data[head % TOTAL_SPACE];

        head++;

        return resultValue;
    }

    /**
     * Overrides the method claimed in Object, the superclass of any class.
     * @return
     */
    @Override
    public String toString() {
        String resultString = "";

        if (head == tail) {
            return "empty";
        }

        for (int i = head; i < tail; i++) {
            resultString += data[i % TOTAL_SPACE] + ", ";
        }

        return resultString;
    }

    public static void main(String args[]) {
        CircleCharQueue tempQueue = new CircleCharQueue();
        System.out.println("Initialized, the list is: " + tempQueue.toString());

        for (char i = '0'; i < '5'; i++) {
            tempQueue.enqueue(i);
        }
        System.out.println("Enqueue, the queue is: " + tempQueue.toString());

        char tempValue = tempQueue.dequeue();
        System.out.println("Dequeue " + tempValue + ", the queue is: " + tempQueue.toString());

        for (char i = 'a'; i < 'f'; i++) {
            tempQueue.enqueue(i);
            System.out.println("Enqueue, the queue is: " + tempQueue.toString());
        }

        for (int i = 0; i < 3; i++) {
            tempValue = tempQueue.dequeue();
            System.out.println("Dequeue " + tempValue + ", the queue is: " + tempQueue.toString());
        }

        for (char i = 'A'; i < 'F'; i++) {
            tempQueue.enqueue(i);
            System.out.println("Enqueue, the queue is: " + tempQueue.toString());
        }
    }

}