队列的思想

先进先出

队列的基本思想.jpeg

应用场景

消息队列，系统任务

队列实现 Java

基本API：isEmpty，size，enQueue，deQueue

package struct;

import java.util.Iterator;

public class Queue<T> implements Iterable<T> {
    private Node first;
    private Node last;
    private int N;

    public int size() {
        return N;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public void enQueue(T val) {
        Node previous = last;
        last = new Node();
        last.val = val;
        last.next = null;
        if (isEmpty()) {
            first = last;
        } else {
            previous.next = last;
        }
        N++;
    }

    public T deQueue() {
        T val = first.val;
        first = first.next; // 最后一个时，first = first.next = null
        // 判断条件是first == null，其实也可以用 N == 0，不过这就要把N--写在前面
        if (isEmpty()) {
            last = null; // first为null时，last也要为null才合理
        }
        N--;
        return val;
    }

    @Override
    public Iterator<T> iterator() {
        return new QueueIterator();
    }
    // 内部类-----------------------------
    private class Node {
        T val;
        Node next;
    }

    private class QueueIterator implements Iterator<T> {
        Node curr = first;
        @Override
        public boolean hasNext() {
            return curr != null;
        }

        @Override
        public T next() {
            T val = curr.val;
            curr = curr.next;
            return val;
        }
    }

    // 测试类--------------------------------
    //测试类---------------------------------------
    public static class Test {
        public static void main(String[] args) {
            Queue<Integer> queue = new Queue<>();
            queue.enQueue(1);
            queue.enQueue(2);
            queue.enQueue(3);
            queue.enQueue(4);
            queue.enQueue(5);

            Iterator iterator = queue.iterator();
            while (iterator.hasNext()) {
                System.out.println(iterator.next());
            }

            int size = queue.size();
            for (int i = 0; i < size; i++) {
                System.out.println(queue.deQueue());
            }
        }
    }
}