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package queue;

public class ArrayQueue<T> implements Queue<T> {

private T[] items;

private int capacity;

private int head;

private int tail;

public ArrayQueue(int capacity) {

this.capacity = capacity;

this.items = (T[]) new Object[capacity];

this.head = 0;

this.tail = 0;

}

// T(N)=O(1)

@Override

public boolean enqueue(T item) {

if (tail == capacity) {

// head == 0 && tail == capacity, queue is full.

if (head == 0) {

return false;

}

// 数据搬移，空出位置

for (int i = head; i < tail; i++) {

items[i-head] = items[i];

}

// 搬移完之后，更新head和tail

tail -= head;

head = 0;

}

items[tail] = item;

tail++;

return true;

}

// T(N)=O(1)

@Override

public T dequeue() {

if (head == tail) {

return null;

}

T tmp = items[head];

head++;

return tmp;

}

// for test

public void printAll() {

System.out.print("[");

for (int i = head; i < tail; i++) {

System.out.print(items[i] + " ");

}

System.out.print("]");

System.out.println();

}

public static void main(String[] args) {

ArrayQueue<String> queue = new ArrayQueue<>(4);

queue.printAll();

queue.enqueue("A");

queue.printAll();

queue.enqueue("B");

queue.enqueue("C");

queue.enqueue("D");

queue.printAll();

queue.enqueue("E");

queue.printAll();

queue.dequeue();

queue.dequeue();

queue.dequeue();

queue.printAll();

}

}