设计一个循环队列QUEUE<T>，用data[0..MaxSize-1]存放队列元素，用front和rear分别作为队头和队尾指针，用tag标识队列可能空（false）或满（true），要求设计队列的相关基本运算算法。

template <typename T>
class QUEUE {
private:
    T *data;
    int front, rear, MaxSize;
    bool tag;  // false表示队列未满，true表示队列已满

public:
    QUEUE(int MaxQueueSize) {
        data = new T[MaxQueueSize];
        MaxSize = MaxQueueSize;
        front = rear = 0;
        tag = false;
    }

    ~QUEUE() {
        delete[] data;
    }

    void EnQueue(T x) {
        if (tag) {
            cout << "Queue is Full.\n";
            return;
        }
        data[rear] = x;
        rear = (rear + 1) % MaxSize;
        if (rear == front)
            tag = true;
    }

    T DeQueue() {
        if (!tag) {
            cout << "Queue is Empty.\n";
            return NULL;
        }
        T x = data[front];
        front = (front + 1) % MaxSize;
        if (rear == front)
            tag = false;
        return x;
    }

    bool IsEmpty() {
        return (rear == front) && (!tag);
    }

    bool IsFull() {
        return tag;
    }

    void ClearQueue() {
        front = rear = 0;
        tag = false;
    }

    int QueueLength() {
        if (tag)
            return MaxSize;
        else
            return (rear - front + MaxSize) % MaxSize;
    }

    T GetHead() {
        if (!IsEmpty())
            return data[front];
        else
            return NULL;
    }
};