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package queue;

public class CircularQueue<T> {

private T[] items;

private int capacity;

private int head;

private int tail;

public CircularQueue(int capacity) {

this.capacity = capacity;

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

this.head = 0;

this.tail = 0;

}

// T(N)=O(1)

public boolean enqueue(T item) {

// 队满条件

if ((tail + 1) % capacity == head) {

return false;

}

items[tail] = item;

tail = (tail + 1) % capacity;

return true;

}

// T(N)=O(1)

public T dequeue() {

// 队空条件

if (head == tail) {

return null;

}

T tmp = items[head];

items[head] = null; // 这里可以选择置为nulL,也可以不置为null.

head = (head + 1) % capacity;

return tmp;

}

// for test

public void printAll() {

System.out.print("[");

for (int i = 0; i < capacity; i++) {

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

}

System.out.print("]");

System.out.println();

}

public static void main(String[] args) {

CircularQueue<java.lang.String> queue = new CircularQueue<>(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.enqueue("E");

queue.printAll();

}

}