队列

API

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import java.util.Iterator;

public class Queue<E> implements Iterable<E> {
    
    /**
     * 入队
     */
    public void enqueue(E e);
    
    /**
     * 出队，若队列为空，这抛出 IllegalArgumentException
     */
    public E dequeue();
    
    /**
     * 元素个数
     */
    public int size();
    
    /**
     * 是否为空
     */
    public boolean isEmpty();

    /**
     * 迭代器
     */
    @Override
    public Iterator<E> iterator();
}

实现

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public class Queue<E> implements Iterable<E> {

    private static class Node<T> {
        T item;
        Node<T> next;

        Node(T item, Node<T> next) {
            this.item = item;
            this.next = next;
        }
    }

    private Node<E> head;

    private Node<E> tail;

    private int size;

    /**
     * 入队
     */
    public void enqueue(E e) {
        Node<E> node = new Node<>(e, null);
        if (head == null) {
            head = node;
            tail = node;
        } else {
            tail.next = node;
            tail = node;
        }
        size++;
    }

    /**
     * 出队，若队列为空，这抛出 IllegalArgumentException
     */
    public E dequeue() {
        if (isEmpty()) {
            throw new IllegalArgumentException("empty queue");
        }
        Node<E> node = head;
        head = head.next;
        node.next = null; // help gc
        size--;
        return node.item;
    }

    /**
     * 元素个数
     */
    public int size() {
        return size;
    }

    /**
     * 是否为空
     */
    public boolean isEmpty() {
        return head == null;
    }

    private class QueueIterator implements Iterator<E> {
        Node<E> current = head;

        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public E next() {
            Node<E> node = current;
            current = current.next;
            return node.item;
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new QueueIterator();
    }

}

测试

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public static void main(String[] args) {
    Queue<Integer> queue = new Queue<>();
    for (int i = 0; i < 5; i++) {
        queue.enqueue(i);
    }
    System.out.println("queue size is : " + queue.size());

    for (int i : queue) {
        System.out.println(i);
    }

    for (int i = 0; i < 5; i++) {
        queue.dequeue();
    }
    System.out.println("queue size is : " + queue.size());
    System.out.println("queue is empty : " + queue.isEmpty());
}